Expression and genetic analysis of prtb, a gene that encodes a highly conserved proline-rich protein expressed in the brain

DAZAP2 acts as specifier of the p53 response to DNA damage

The DNA damage-responsive tumor suppressors p53 and HIPK2 are well established regulators of cell fate decision-making and regulate the cellular sensitivity to DNA-damaging drugs. Here, we identify Deleted in Azoospermia-associated protein 2 (DAZAP2), a small adaptor protein, as a novel regulator of HIPK2 and specifier of the DNA damage-induced p53 response. Knock-down or genetic deletion of DAZAP2 strongly potentiates cancer cell chemosensitivity both in cells and in vivo using a mouse tumour xenograft model. In unstressed cells, DAZAP2 stimulates HIPK2 polyubiquitination and degradation through interplay with the ubiquitin ligase SIAH1. Upon DNA damage, HIPK2 site-specifically phosphorylates DAZAP2, which terminates its HIPK2-degrading function and triggers its re-localization to the cell nucleus. Interestingly, nuclear DAZAP2 interacts with p53 and specifies target gene expression through modulating a defined subset of p53 target genes. Furthermore, our results suggest that DAZAP2 co-occupies p53 response elements to specify target gene expression. Collectively, our findings propose DAZAP2 as novel regulator of the DNA damage-induced p53 response that controls cancer cell chemosensitivity.

Epigenome-wide association study of depression symptomatology in elderly monozygotic twins

Depression is a severe and debilitating mental disorder diagnosed by evaluation of affective, cognitive and physical depression symptoms. Severity of these symptoms strongly impacts individual's quality of life and is influenced by a combination of genetic and environmental factors. One of the molecular mechanisms allowing for an interplay between these factors is DNA methylation, an epigenetic modification playing a pivotal role in regulation of brain functioning across lifespan. The aim of this study was to investigate if there are DNA methylation signatures associated with depression symptomatology in order to identify molecular mechanisms contributing to pathophysiology of depression. We performed an epigenome-wide association study (EWAS) of continuous depression symptomatology score measured in a cohort of 724 monozygotic Danish twins (346 males, 378 females). Through EWAS analyses adjusted for sex, age, flow-cytometry based blood cell composition, and twin relatedness structure in the data we identified depression symptomatology score to be associated with blood DNA methylation levels in promoter regions of neuropsin (KLK8, p-value = 4.7 × 10^-7) and DAZ associated protein 2 (DAZAP2, p-value = 3.13 × 10^-8) genes. Other top associated probes were located in gene bodies of MAD1L1 (p-value = 5.16 × 10^-6), SLC29A2 (p-value = 6.15 × 10^-6) and AKT1 (p-value = 4.47 × 10^-6), all genes associated before with development of depression. Additionally, the following three measures (a) DNAmAge (calculated with Horvath and Hannum epigenetic clock estimators) adjusted for chronological age, (b) difference between DNAmAge and chronological age, and (c) DNAmAge acceleration were not associated with depression symptomatology score in our cohort. In conclusion, our data suggests that depression symptomatology score is associated with DNA methylation levels of genes implicated in response to stress, depressive-like behaviors, and recurrent depression in patients, but not with global DNA methylation changes across the genome.

Promoter methylation induced epigenetic silencing of DAZAP2, a downstream effector of p38/MAPK pathway, in multiple myeloma cells

Multiple myeloma (MM) is hematological malignancy characterized by clonal proliferation of malignant plasma cells in the bone marrow environment. Previously, we identified DAZAP2 as a candidate cancer suppressor gene, the downregulation of which is regulated by its own promoter methylation status. In the current study, we analyzed the DAZAP2 promoter in MM cell lines KM3, MM.1S, OPM-2, and ARH77 by bisulfite genomic sequencing assay. We identified the binding site for transcription factor cyclic adenosine monophosphate response element binding (CREB) in the DAZAP2 promoter CpG2, and we found that hypermethylation of the CREB binding motif in the DAZAP2 promoter is responsible for the reduced DAZAP2 expression in MM cells. Later we checked the p38/MAPK signaling cascade, which is reported to regulate expression and function of CREB. Our results showed that the p38/MAPK signaling pathway drives the expression of DAZAP2 by phosphorylation of CREB, and hypermethylation of CREB binding motif in DAZAP2 promoter can inhibit binding of CREB to the latter, thus downregulating DAZAP2 expression. Moreover, treating the MM cells with 5-aza-2' deoxycytidine to demethylate DAZAP2 promoter restored the binding of CREB to its binding motif, and thus upregulated DAZAP2 expression. Our results not only identified DAZAP2 as a new downstream target of p38/MAPK/CREB signaling cascade, but we also clarified that the downregulation of DAZAP2 in MM cells is caused by hypermethylation of CREB binding motif in its own promoter region, which implies that demethylation of DAZAP2 promoter can be a novel therapeutic strategy for MM treatment.

Spatial and temporal inhibition of FGFR2b ligands reveals continuous requirements and novel targets in mouse inner ear morphogenesis

Morphogenesis of the inner ear epithelium requires coordinated deployment of several signaling pathways, and disruptions cause abnormalities of hearing and/or balance. The FGFR2b ligands FGF3 and FGF10 are expressed throughout otic development and are required individually for normal morphogenesis, but their prior and redundant roles in otic placode induction complicates investigation of subsequent combinatorial functions in morphogenesis. To interrogate these roles and identify new effectors of FGF3 and FGF10 signaling at the earliest stages of otic morphogenesis, we used conditional gene ablation after otic placode induction, and temporal inhibition of signaling with a secreted, dominant-negative FGFR2b ectodomain. We show that both ligands are required continuously after otocyst formation for maintenance of otic neuroblasts and for patterning and proliferation of the epithelium, leading to normal morphogenesis of both the cochlear and vestibular domains. Furthermore, the first genome-wide identification of proximal targets of FGFR2b signaling in the early otocyst reveals novel candidate genes for inner ear development and function.

Evolutionary features and intracellular behavior of the PRTB protein

Human PRTB encodes a proline-rich protein of 168 amino acids (PRTB). We analyzed the evolutionary patterns of PRTB from various vertebrate species. Maximum likelihood analyses indicated that while mammalian PRTB has been very well conserved and underwent a significantly slower rate of evolution, only the branch leading to fish PRTB has undergone adaptive evolution. We generated several mutant PRTBs fused to the GFP variant, Venus, and found that the degradation of PRTB was enhanced by the transfection of an E2, UbcH5. Since mutation of the K153 site in PRTB was refractory to its degradation, proteolysis was suggested to be mediated by ubiquitination of K153. The subcellular localization of PRTB was also investigated, which showed that mutation of the K4 site completely prevented the nuclear localization of this protein. Together, these results suggest that Lys residues might play important roles in regulating the intracellular dynamics of the PRTB protein.

Dazap2 is required for FGF-mediated posterior neural patterning, independent of Wnt and Cdx function

The organization of the embryonic neural plate requires coordination of multiple signal transduction pathways, including fibroblast growth factors (FGFs), bone morphogenetic proteins (BMPs), and WNTs. Many studies have suggested that a critical component of this process is the patterning of posterior neural tissues by an FGF-caudal signaling cascade. Here, we have identified a novel player, Dazap2, and show that it is required in vivo for posterior neural fate. Loss of Dazap2 in embryos resulted in diminished expression of hoxb9 with a concurrent increase in the anterior marker otx2. Furthermore, we found that Dazap2 is required for FGF dependent posterior patterning; surprisingly, this is independent of Cdx activity. Furthermore, in contrast to FGF activity, Dazap2 induction of hoxb9 is not blocked by loss of canonical Wnt signaling. Functionally, we found that increasing Dazap2 levels alters neural patterning and induces posterior neural markers. This activity overcomes the anteriorizing effects of noggin, and is downstream of FGF receptor activation. Our results strongly suggest that Dazap2 is a novel and essential branch of FGF-induced neural patterning.

Dazap2 modulates transcription driven by the Wnt effector TCF-4

A major outcome of the canonical Wnt/beta-catenin-signalling pathway is the transcriptional activation of a specific set of target genes. A typical feature of the transcriptional response induced by Wnt signalling is the involvement of Tcf/Lef factors that function in the nucleus as the principal mediators of signalling. Vertebrate Tcf/Lef proteins perform two well-characterized functions: in association with beta-catenin they activate gene expression, and in the absence of Wnt ligands they bind TLE/Groucho proteins to act as transcriptional repressors. Although the general characteristics of Tcf/Lef factors are well understood, the mechanisms that control their specific roles in various cellular backgrounds are much less defined. In this report we reveal that the evolutionary conserved Dazap2 protein functions as a TCF-4 interacting partner. We demonstrate that a short region proximal to the TCF-4 HMG box mediates the interaction and that all Tcf/Lef family members associate with Dazap2. Interestingly, knockdown of Dazap2 not only reduced the activity of Wnt signalling as measured by Tcf/beta-catenin reporters but additionally altered the expression of Wnt-signalling target genes. Finally, chromatin immunoprecipitation studies indicate that Dazap2 modulates the affinity of TCF-4 for its DNA-recognition motif.

Proline-rich transcript in brain protein induces stress granule formation

The repression of translation in environmentally stressed eukaryotic cells causes the sequestration of translation initiation factors and the 40S ribosomal subunit into discrete cytoplasmic foci called stress granules (SGs). Most components of the preinitiation complex, such as eIF3, eIF4A, eIF4E, eIF4G, and poly(A)-binding protein, congregate into SGs under stress conditions. However, the molecular basis of translation factor sequestration into SGs has not been clearly elucidated. Here, we report that proline-rich transcript in brain (PRTB) protein interacts with eIF4G and participates in SG formation. PRTB was recruited to SG under sodium arsenite and heat stress conditions. When overexpressed, PRTB inhibited global translation and formed SGs containing TIA-1, eIF4G, and eIF3. Knockdown of PRTB reduced the SG formation induced by sodium arsenite. These results suggest that PRTB not only is a component of SG formed by cellular stresses but also plays an important role in SG formation via an interaction with the scaffold protein eIF4G, which is associated with many translation factors and mRNAs.

The structure, expression and function prediction of DAZAP2, a down-regulated gene in multiple myeloma

In our previous studies, DAZAP2 gene expression was down-regulated in untreated patients of multiple myeloma (MM). For better studying the structure and function of DAZAP2, a full-length cDNA was isolated from mononuclear cells of a normal human bone marrow, sequenced and deposited to Genbank (AY430097). This sequence has an identical ORF (open reading frame) as the NM_014764 from human testis and the D31767 from human cell line KG-1. Phylogenetic analysis and structure prediction reveal that DAZAP2 homologues are highly conserved throughout evolution and share a polyproline region and several potential SH2/SH3 binding sites. DAZAP2 occurs as a single-copy gene with a four-exon organization. We further noticed that the functional DAZAP2 gene is located on Chromosome 12 and its pseudogene gene is on Chromosome 2 with electronic location of human chromosome in Genbank, though no genetic abnormalities of MM have been reported on Chromosome 12. The ORF of human DAZAP2 encodes a 17-kDa protein, which is highly similar to mouse Prtb. The DAZAP2 protein is mainly localized in cytoplasm with a discrete pattern of punctuated distribution. DAZAP2 may associate with carcinogenesis of MM and participate in yet-to-be identified signaling pathways to regulate proliferation and differentiation of plasma cells.

Codominant expression of genes coding for different sets of inducible salivary polypeptides associated with parotid hypertrophy in two inbred mouse strains

Experimental mouse parotid hypertrophy has been associated with the expression of a number of isoproterenol-induced salivary proline-rich polypeptides (IISPs). Mouse salivary proline-rich proteins (PRPs) have been mapped both to chromosomes 6 and 8. Recently, mice of two inbred strains (A/Snell and A. Swiss) have been found to differ drastically in the IISPs. In this study, mice of both strains were used for cross-breeding experiments addressed to define the pattern of inheritance of the IISP phenotype and to establish whether the IISPs are coded on a single or on several chromosomes. The IISP phenotype of individual mice was assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of whole saliva collected after three daily stimulations by isoproterenol. Parental A/Snell and A. Swiss mice were homogeneous for distinctive strain-associated IISP-patterns. First filial generation (F1) mice obtained from the cross of A/Snell with A. Swiss mice expressed with no exception both the A/Snell and A. Swiss IISPs (coexpression). In the second filial generation (F2) both parental IISP phenotypes reappeared together with a majority of mice expressing the F1-hybrid phenotype (1:2:1 ratio). Backcrosses of F1 x A/Snell and F1 x A. Swiss produced offsprings displaying the F1 and the corresponding parental phenotypes with a 1:1 ratio. No recombinants were observed among F2 mice or among mice resulting from backcrosses. Thus, genes coding for the IISPs that are expressed differentially in both mouse strains are located on the same chromosome, probably at the same locus (alleles) or at quite closely linked loci (nonalleles).

Dicer is required for embryonic angiogenesis during mouse development

Dicer is a multi-domain protein responsible for the generation of short interfering RNAs (siRNAs) from long double-stranded RNAs during RNA interference. It is also involved in the maturation of microRNAs, some of which are transcriptional regulators of developmental timing in nematodes. To assess the role of Dicer in mammals, we generated Dicerex1/2 mice with a deletion of the amino acid sequences corresponding to the first and second exons of the dicer gene via homologous recombination. We found that Dicerex1/2 homozygous embryos displayed a retarded phenotype and died between days 12.5 and 14.5 of gestation. Thus, these results show that dicerex1/2 is severely hypomorphic and that Dicer is essential for normal mouse development. Interestingly, we also found that blood vessel formation/maintenance in dicerex1/2 embryos and yolk sacs were severely compromised, suggesting a possible role for Dicer in angiogenesis. This finding is consistent with the altered expression of vegf, flt1, kdr, and tie1 in the mutant embryos. Taken together, the results of this study indicate that Dicer exerts its function on mouse embryonic angiogenesis probably through its role in the processing of microRNAs that regulate the expression levels of some critical angiogenic regulators in the cell.

Sox6 regulation of cardiac myocyte development

A mouse mutation (p100H/p100H) has been identified that is associated with cardioskeletal myopathy, heart block, delayed growth and early postnatal death. The gene that is disrupted in this mutation encodes the transcription factor Sox6. P19CL6 cells were used as an in vitro cardiomyocyte differentiation system and revealed that Sox6 is expressed exclusively when the cells are committed to differentiate to beating cardiac myocytes. We used the yeast two-hybrid system to identify the Prtb (Proline-rich transcript of the brain) protein as a Sox6 interactor, and subsequently confirmed the interaction by co-immunoprecipitation. Prtb expression in P19CL6 cells increased with differentiation to beating cardiomyocytes. Using the P19CL6 cells stably transfected with noggin, an antagonist of BMP (Bone Morphogenic Protein), we found that BMP expression is required for Sox6 expression in cardiomyocyte differentiation. Surprisingly, the expression of the alpha1c-subunit gene of the L-type Ca2+ channel decreased in P19CL6 cells as they differentiated to beating cardiac cells. Ectopic expression of Sox6 or Prtb alone in P19CL6 cells caused down-regulation of L-type Ca2+ alpha1c expression, but when Sox6 and Prtb were co-transfected to the cells, L-type Ca2+ alpha1c remained at basal levels. A similar relationship of Sox6 and L-type Ca2+ alpha1c expression was seen in vivo (comparing wild-type and p(100H)/p(100H) mutant mice). Thus, Sox6 is within the BMP pathway in cardiac differentiation, interacts with Prtb and may play a critical role in the regulation of a cardiac L-type Ca2+ channel.

Differential expression of isoproterenol-induced salivary polypeptides in two mouse strains that are congenic for the H-2 histocompatibility gene complex

Two inbred mouse strains, A/Snell and A.Swiss, which were produced as congenic with regard to the H-2 histocompatibility gene complex, are homozygous for two different groups of isoproterenol-induced salivary polypeptides (IISP). These polypeptides, which have been considered as markers of the hypertrophic growth of the parotid acinar cells, are members of the complex family of salivary proline-rich proteins (PRP) on the basis of both their massive accumulation in the parotid acinar cells in response to chronic isoproterenol, secretory character, high solubility in trichloroacetic acid and metachromatic staining by Coomassie blue. IISP expressed in both mouse strains were identified by unidimensional SDS-polyacrylamide electrophoresis and Coomassie blue staining both in parotid gland homogenates and in whole salivas obtained from mice repeatedly stimulated at 24-h intervals with isoproterenol. Parotid glands from 40 mice (20 A/Snell and 20 A.Swiss) and salivas from 270 mice (200 A/Snell and 70 A.Swiss) were analyzed. One of the congenic strains (A/Snell) expressed five IISP (Mr 65, 61, 51.5, 38, and 37 kDa) and the other strain (A.Swiss) expressed six IISP (Mr 59, 57, 54.5, 46, 36, and 34 kDa). No inter-individual intra-strain variations were observed, thus defining strain-associated patterns of IISP (PRP).

Differentiation of pluripotent embryonic stem cells into cardiomyocytes

Embryonic stem (ES) cells have been established as permanent lines of undifferentiated pluripotent cells from early mouse embryos. ES cells provide a unique system for the genetic manipulation and the creation of knockout strains of mice through gene targeting. By cultivation in vitro as 3D aggregates called embryoid bodies, ES cells can differentiate into derivatives of all 3 primary germ layers, including cardiomyocytes. Protocols for the in vitro differentiation of ES cells into cardiomyocytes representing all specialized cell types of the heart, such as atrial-like, ventricular-like, sinus nodal-like, and Purkinje-like cells, have been established. During differentiation, cardiac-specific genes as well as proteins, receptors, and ion channels are expressed in a developmental continuum, which closely recapitulates the developmental pattern of early cardiogenesis. Exploitation of ES cell-derived cardiomyocytes has facilitated the analysis of early cardiac development and has permitted in vitro "gain-of-function" or "loss-of-function" genetic studies. Recently, human ES cell lines have been established that can be used to investigate cardiac development and the function of human heart cells and to determine the basic strategies of regenerative cell therapy. This review summarizes the current state of ES cell-derived cardiogenesis and provides an overview of how genomic strategies coupled with this in vitro differentiation system can be applied to cardiac research.

Proline-rich transcript of the brain (prtb) is a serum-responsive gene in osteoblasts and upregulated during adhesion

To characterize the temporal expression of genes that play a functional role during the process of osteoblast adhesion, we used differential display (DD-PCR) on mRNA isolated from attached vs. suspended osteoblasts. A 200-bp fragment displaying upregulated expression after 30 and 60 min adhesion was isolated, sequenced, and showed 97% homology to prtb, previously showed to be expressed in mouse brain. Northern analysis confirmed a two-fold increase in prtb message during adhesion to tissue culture polystyrene, both in the presence or absence of surface-adsorbed serum proteins. Serum stimulation alone was also able to induce prtb expression, although to a lesser extent, in suspension cells. Strong prtb expression was also detected in both brain and bone of adult rats. Furthermore, prtb expression analysis during MC3T3-E1 cell differentiation revealed high expression levels independent of proliferation (day 0-7), matrix maturation (day 7-14), and mineralization (day 14-31). Time course analysis of prtb expression during adhesion of sensitized osteoblasts to serum-protein coated surfaces showed robust mRNA expression at 5 min post-plating and a peak at 10 min. The two known serum-inducible immediate early genes c-fos and c-jun showed similar expression kinetics, with c-jun mRNA levels peaking at 15 min and c-fos at 20 min. Based on these data, we hypothesize that prtb may function as an immediate early, serum-responsive, and adhesion-inducible gene with possible involvement in processes such as cell cycle control, adhesion, and proliferation.

Impaired motor coordination in mice that lack punc

The punc gene, encoding a member of the neural cell adhesion molecule family expressed in the developing central nervous system, limbs, and inner ear, was identified. To extend studies of the normal expression pattern of punc and to determine its function, a mouse strain bearing a lacZ/neo insertion in a 5' coding exon was created. The complex pattern of punc expression in embryos from embryonic day 9.5 (E9.5) to E11.5 was mimicked accurately by beta-galactosidase (beta-Gal) activity. As development proceeded, the distribution of beta-Gal activity was increasingly restricted, finally becoming confined to the brain and inner ear by E15.5. In the adult, beta-Gal activity was detected in several regions of the inner ear and brain and was particularly strong in the cerebellar Bergmann glia. Genetic analysis of this null allele demonstrated that punc is not required for normal embryogenesis. Interestingly, comparisons of beta-Gal activity and punc transcripts in heterozygous and homozygous mutant individuals demonstrated that punc is negatively autoregulated in some tissues. Adult punc-deficient mice were overtly normal and had normal hearing. Compared with control littermates, however, homozygous mutants had significantly reduced retention times on the Rotarod, suggesting a role for Bergmann glia-expressed Punc in the cerebellar control of motor coordination.

Identification of osmosensitive and ammonia-regulated genes in rat astrocytes by Northern blotting and differential display reverse transcriptase-polymerase chain reaction

BACKGROUND/AIMS

The hepatic encephalopathy (HE) is in part the result of astrocyte swelling with alterations of glial function. Detoxification of ammonia may be one mechanism by which astrocyte swelling is triggered in HE.

METHODS

The differential display polymerase chain reaction (DDRT-PCR) and Northern blot analysis were used for study the functional consequence of cell volume changes and ammonia on gene expression in primary rat astrocytes.

RESULTS

Differentially expressed cDNA products were identified with about 92% homology to genes coding for mouse proline rich protein expressed in brain (PRTB), rat clusterin, elongin, and human Kelch motif containing protein. As shown by Northern blot analysis, PRTB and clusterin mRNA levels were upregulated by 19-64% after 4-8 h by both ammonia and hypoosmolarity. Elongin mRNA expression increased by 97% in response to ammonia but slightly by hypoosmolarity. Further, hypoosmotic exposure for 1-24 hours but not ammonia led to an increase of 80% in KMCP mRNA levels.

CONCLUSIONS

The identification of these genes offers the opportunity to identify unrecognized molecular mechanisms of HE. The finding that several genes are induced by both, hypoosmolarity and ammonia, supports the view that astrocyte swelling is a major, but not the only pathogenetic event in HE.

Nuclear import/export of hRPF1/Nedd4 regulates the ubiquitin-dependent degradation of its nuclear substrates

The ubiquitin-protein ligase (E3), hRPF1/Nedd4, is a component of the ubiquitin-proteasome pathway responsible for substrate recognition and specificity. Although previously characterized as a regulator of the stability of cytoplasmic proteins, hRPF1/Nedd4 has also been suggested to have a role in the nucleus. However, in light of the cytoplasmic localization of hRPF1/Nedd4, it is unclear whether bona fide nuclear substrates of hRPF1/Nedd4 exist, and if so, what mechanism may allow a cytoplasmic ubiquitin ligase to manifest nuclear activity. Our search for nuclear substrates led to the identification of the human proline-rich transcript, brain-expressed (hPRTB) protein, the ubiquitination and degradation of which is regulated by hRPF1/Nedd4. Interestingly, hPRTB colocalizes with the splicing factor SC35 in nuclear speckles. Finally, we demonstrate that hRPF1/Nedd4 is indeed capable of entering the nucleus; however, the presence of a functional Rev-like nuclear export sequence in hRPF1/Nedd4 ensures a predominant cytoplasmic localization. Cumulatively, these findings highlight a nuclear role for the ubiquitin ligase hRPF1/Nedd4 and underscore cytoplasmic/nuclear localization as an important regulatory component of hRPF1/Nedd4-substrate recognition.

Identification of genes induced by inflammatory cytokines in airway epithelium

Epithelial cells lining the airways are thought to play a prominent role in respiratory diseases. We utilized cDNA representational difference analysis to identify the genes in which expression is induced by the proinflammatory cytokines tumor necrosis factor-alpha and interleukin-1beta in primary human bronchial epithelial cells and hence are relevant to airway inflammation. Hybridization of the subtraction product to arrayed cDNAs indicated that known tumor necrosis factor-alpha- and interleukin-1beta-inducible genes such as B94, Zfp36, and regulated on activation normal T cell expressed and secreted were represented, confirming the success of the subtraction experiment. A 1,152-clone library potentially representing genes with higher transcript levels in cytokine-treated human bronchial epithelial cells was generated and sequenced. Sequence similarity searches indicated that these clones represented 57 genes of known function, 1 gene of unknown function, 6 expressed sequence tags, and 2 novel sequences. The expression of 19 of these clones was studied by a combination of Northern blotting and RT-PCR analyses and confirmation of differential expression for 10 known genes, 2 expressed sequence tags, and a novel sequence not represented in any of the public databases was obtained. Thus cDNA representational difference analysis was utilized to isolate known and novel differentially expressed genes, which putatively play a role in airway inflammation.

Defective organellar membrane protein trafficking in Ap3b1-deficient cells

AP-3 is a heterotetrameric protein complex involved in intracellular vesicle transport. Molecular analyses show that Ap3b1, which encodes the AP-3 (&bgr;)3A subunit, is altered in pearl mice. To provide genetic evidence that mutation of Ap3b1 is responsible for the pearl phenotype and to determine the null phenotype, the Ap3b1 gene was disrupted by homologous recombination. Mice homozygous for the resulting allele, Ap3b1(LN), or compound heterozygotes with pearl, displayed phenotypes similar to those of pearl mice, confirming that Ap3b1 is the causal gene for pearl. Moreover, pearl is likely to be a hypomorph as the Ap3b1(LN) homozygotes had a lighter coat color and accumulated fewer of the micro3 and (&dgr;)3 subunits of AP-3 than did pearl mice. Finally, immunofluorescence analysis of fibroblasts and melanocytes cultured from Ap3b1(LN) homozygotes revealed that the lysosomal membrane proteins Lamp I and Lamp II and the melanosomal membrane protein tyrosinase were mislocalized. In particular, the Lamp proteins were clustered on the cell surface. These findings strengthen the evidence for an alternate pathway via the plasma membrane for cargo normally transported to organelles by AP-3.