Mechanisms of translocation of proteins across membranes

Improved trafficking and expression of luminopsins for more efficient optical and pharmacological control of neuronal activity

Luminopsins (LMOs) are chimeric proteins consisting of a luciferase fused to an opsin that provide control of neuronal activity, allowing for less cumbersome and less invasive optogenetic manipulation. It was previously shown that both an external light source and the luciferase substrate, coelenterazine (CTZ), could modulate activity of LMO-expressing neurons, although the magnitudes of the photoresponses remained subpar. In this study, we created an enhanced iteration of the excitatory luminopsin LMO3, termed eLMO3, that has improved membrane targeting due to the insertion of a Golgi trafficking signal sequence. In cortical neurons in culture, the expression of eLMO3 resulted in significant reductions in the formation of intracellular aggregates, as well as in a significant increase in total photocurrents. Furthermore, we corroborated the findings with injections of adeno-associated viral vectors into the deep layers of the somatosensory cortex (the barrel cortex) of male mice. We observed greatly reduced numbers of intracellular puncta in eLMO3-expressing cortical neurons compared to those expressing the original LMO3. Finally, we quantified CTZ-driven behavior, namely whisker-touching behavior, in male mice with LMO3 expression in the barrel cortex. After CTZ administration, mice with eLMO3 displayed significantly longer whisker responses than mice with LMO3. In summary, we have engineered the superior LMO by resolving membrane trafficking defects, and we demonstrated improved membrane targeting, greater photocurrents, and greater functional responses to stimulate with CTZ.

Roles for ATF6 and the sarco/endoplasmic reticulum protein quality control system in the heart

The hypertrophic growth of cardiac myocytes is a highly dynamic process that underlies physiological and pathological adaptation of the heart. Accordingly, a better understanding of the molecular underpinnings of cardiac myocyte hypertrophy is required in order to fully appreciate the causes and functional consequences of the changes in the size of the healthy and diseased heart. Hypertrophy is driven by increases in cardiac myocyte protein, which must be balanced by cellular ability to maintain protein quality in order to avoid maladaptive accumulation of toxic misfolded proteins. Recent studies have shown that the endoplasmic reticulum (ER), which, in cardiac myocytes, comprises the sarco/endoplasmic reticulum (SR/ER), is the site of most protein synthesis. Thus, the protein quality control machinery located at the SR/ER is likely to be an important determinant of whether the heart responds adaptively to hypertrophic growth stimuli. The SR/ER-transmembrane protein, ATF6, serves a critical protein quality control function as a first responder to the accumulation of potentially toxic, misfolded proteins. Misfolded proteins transform ATF6 into a transcription factor that regulates a gene program that is partly responsible for enhancing protein quality control. Two ATF6-inducible genes that have been studied in the heart and shown to be adaptive are RCAN1 and Derl3, which encode proteins that decrease protein-folding demand, and enhance degradation of misfolded proteins, respectively. Thus, the ATF6-regulated SR/ER protein quality control system is important for maintaining protein quality during growth, making ATF6, and other components of the system, potentially attractive targets for the therapeutic management pathological cardiac hypertrophy. This article is part of a Special Issue entitled "Protein Quality Control, the Ubiquitin Proteasome System, and Autophagy".

Molecular and cellular approaches for diversifying and extending optogenetics

Optogenetic technologies employ light to control biological processes within targeted cells in vivo with high temporal precision. Here, we show that application of molecular trafficking principles can expand the optogenetic repertoire along several long-sought dimensions. Subcellular and transcellular trafficking strategies now permit (1) optical regulation at the far-red/infrared border and extension of optogenetic control across the entire visible spectrum, (2) increased potency of optical inhibition without increased light power requirement (nanoampere-scale chloride-mediated photocurrents that maintain the light sensitivity and reversible, step-like kinetic stability of earlier tools), and (3) generalizable strategies for targeting cells based not only on genetic identity, but also on morphology and tissue topology, to allow versatile targeting when promoters are not known or in genetically intractable organisms. Together, these results illustrate use of cell-biological principles to enable expansion of the versatile fast optogenetic technologies suitable for intact-systems biology and behavior.

Translocon pores in the endoplasmic reticulum are permeable to small anions

Contribution of translocon peptide channels to the permeation of low molecular mass anions was investigated in rat liver microsomes. Puromycin, which purges translocon pores of nascent polypeptides, creating additional empty pores, raised the microsomal uptake of radiolabeled UDP-glucuronic acid, while it did not increase the uptake of glucose-6-phosphate or glutathione. The role of translocon pores in the transport of small anions was also investigated by measuring the effect of puromycin on the activity of microsomal enzymes with intraluminal active sites. The mannose-6-phosphatase activity of glucose-6-phosphatase and the activity of UDP-glucuronosyltransferase were elevated upon addition of puromycin, but glucose-6-phosphatase and beta-glucuronidase activities were not changed. The increase in enzyme activities was due to a better access of the substrates to the luminal compartment rather than to activation of the enzymes. Antibody against Sec61 translocon component decreased the activity of UDP-glucuronosyltransferase and antagonized the effect of puromycin. Similarly, the addition of the puromycin antagonist anisomycin or treatments of microsomes, resulting in the release of attached ribosomes, prevented the puromycin-dependent increase in the activity. Mannose-6-phosphatase and UDP-glucuronosyltransferase activities of smooth microsomal vesicles showed higher basal latencies that were not affected by puromycin. In conclusion, translationally inactive, ribosome-bound translocons allow small anions to cross the endoplasmic reticulum membrane. This pathway can contribute to the nonspecific substrate supply of enzymes with intraluminal active centers.

Electrophysiological approaches to the study of protein translocation in mitochondria

Electrophysiological techniques have been integral to our understanding of protein translocation across various membranes, and, in particular, the mitochondrial inner and outer membranes. Descriptions of various methodologies (for example, patch clamp, planar bilayers, and tip dip, and their past and potential contributions) are detailed within. The activity of protein import channels of native mitochondrial inner and outer membranes can be studied by directly patch clamping mitochondria and mitoplasts (mitochondria stripped of their outer membrane by French pressing) from various genetically manipulated strains of yeast and mammalian tissue cultured cells. The channel activities of TOM, TIM23, and TIM22 complexes are compared with those reconstituted in proteoliposomes and with those of the recombinant proteins Tom40p, Tim23p, and Tim22p, which play major roles in protein translocation. Studies of the mechanism(s) and the role of channels in protein translocation in mitochondria are prototypes, as the same principles are likely followed in all biological membranes including the endoplasmic reticulum and chloroplasts. The ability to apply electrophysiological techniques to these channels is now allowing investigations into the role of mitochondria in diverse fields such as neurotransmitter release, long-term potentiation, and apoptosis.

The M-PMV cytoplasmic targeting-retention signal directs nascent Gag polypeptides to a pericentriolar region of the cell

Intracytoplasmic protein targeting in mammalian cells is critical for organelle function as well as virus assembly, but the signals that mediate it are poorly defined. We show here that Mason-Pfizer monkey virus specifically targets Gag precursor proteins to the pericentriolar region of the cytoplasm in a microtubule dependent process through interactions between a short peptide signal, known as the cytoplasmic targeting-retention signal, and the dynein/dynactin motor complex. The Gag molecules are concentrated in pericentriolar microdomains, where they assemble to form immature capsids. Depletion of Gag from this region by cycloheximide treatment, coupled with the presence of ribosomal clusters that are in close vicinity to the assembling capsids, suggests that the dominant N-terminal cytoplasmic targeting-retention signal functions in a cotranslational manner. Transport of the capsids out of the pericentriolar assembly site requires the env-gene product, and a functional vesicular transport system. A single point mutation that renders the cytoplasmic targeting-retention signal defective abrogates pericentriolar targeting of Gag molecules. Thus the previously defined cytoplasmic targeting-retention signal appears to act as a cotranslational intracellular targeting signal that concentrates Gag proteins at the centriole for assembly of capsids.

The protein import motor of mitochondria

Proteins that are destined for the matrix of mitochondria are transported into this organelle by two translocases: the TOM complex, which transports proteins across the outer mitochondrial membrane; and the TIM23 complex, which gets them through the inner mitochondrial membrane. Two models have been proposed to explain how this protein-import machinery works -- a targeted Brownian ratchet, in which random motion is translated into vectorial motion, or a 'power stroke', which is exerted by a component of the import machinery. Here, we review the data for and against each model.

The protein import motor of mitochondria: a targeted molecular ratchet driving unfolding and translocation

Unfolding and import of preproteins into mitochondria are facilitated by a molecular motor in which heat shock protein 70 (Hsp70) in the matrix plays an essential role. Here we present two different experimental approaches to analyze mechanisms underlying this function of Hsp70. First, preproteins containing stretches of glutamic acid (polyE) or glycine (polyG) repeats in front of folded domains were imported into mitochondria. This occurred although Hsp70 cannot pull on these stretches to unfold the folded domains, since it does not bind to polyE and polyG. Secondly, preproteins containing titin immunoglobulin (Ig)-like domains were imported into mitochondria, despite the fact that forces of >200 pN are required to mechanically unfold these domains. Since molecular motors generate forces of approximately 5 pN, Hsp70 could not promote unfolding of the Ig-like domains by mechanical pulling. Our observations suggest that Hsp70 acts as an element of a Brownian ratchet, which mediates unfolding and translocation of preproteins across the mitochondrial membranes.

The engagement of Sec61p in the ER dislocation process

Sec61p comprises the endoplasmic reticulum (ER) channel through which nascent polypeptides are imported and from which malfolded proteins have been suggested to be exported, or dislocated, back to the cytoplasm. We have devised a genetic screen for dislocation-specific mutant alleles of SEC61 from S. cerevisiae by employing the unfolded protein response to report on the accumulation of misfolded proteins in the ER. Three of the isolated sec61 alleles are fully proficient in protein translocation into the ER, but defective in the elimination of a misfolded ER luminal substrate and a short-lived ER membrane-spanning model protein, which are otherwise rapidly degraded by cytoplasmic proteolysis in wild-type cells. Our results point to the fourth luminal loop and third transmembrane domain of Sec61p that markedly influence dislocation. We suggest that distinct features of the Sec61-translocon direct the two-way translocation processes.

Molecular characterization of dendritically localized transcripts encoding MAP2

Transcripts encoding high molecular weight (Hwt) isoforms of microtubule-associated protein 2 (MAP2) have been localized in the dendritic compartment of neurons. In contrast, nearly all other neuronal messages, including transcripts encoding low molecular weight (Lwt) MAP2 isoforms, are restricted to cell somas. The mechanisms underlying the dendritic localization of Hwt-MAP2 transcripts are not known. In non-neuronal systems, mRNAs, are localized via signal sequences situated in their 3' untranslated regions (3' UTRs). In this study, we have localized the putative dendritic targeting element (DTE) in Hwt-MAP2 mRNAs by comparing the nucleotide sequences of the somatically localized 6 kb Lwt-MAP2 transcripts with the dendritcally localized 9 kb messages. Our analysis shows that both 6 kb and 9 kb transcripts have identical 3' - and 5'- UTRs, precluding the possibility that the DTE lies in these regions. Within the coding region a single segment that is unique to 9 kb Hwt MAP2 transcripts was identified. These findings suggest that the DTE lies within the 4 kb RNA segment that encodes the projection domain of Hwt-MAP2.

A neuroendocrine-specific protein localized to the endoplasmic reticulum by distal degradation

Regulated endocrine-specific protein, 18-kDa (RESP18), was previously cloned from rat neurointermediate pituitary based on its coordinate regulation with proopiomelanocortin and neuroendocrine specificity. RESP18 has no homology to any known protein. Although RESP18 is translocated across microsomal membranes after in vitro translation, AtT-20 pituitary tumor cells, which endogenously synthesize RESP18, do not release it into the culture medium. In this work, immunostaining and subcellular fractionation have identified RESP18 as an endoplasmic reticulum (ER) protein. Biosynthetic labeling and temperature block studies of AtT-20 cells demonstrated the localization of RESP18 to the ER lumen by a unique mechanism, degradation by proteolysis in a post-ER pre-Golgi compartment. Proteases in this compartment were saturated by exogenous RESP18 overexpression in AtT-20 cells. Furthermore, a calpain protease inhibitor enhanced secretion of RESP18 from AtT-20 cells overexpressing RESP18. Saturation and inhibition of the RESP18 degrading proteases allowed RESP18 to enter secretory granules and acquire a post-translational modification, likely O-glycosylation; this modified 21-kDa RESP18 isoform was the only RESP18 secreted. Rat anterior pituitary extracts contain 18-kDa and O-glycosylated RESP18 with similar properties. Exogenous RESP18 expression in hEK-293 cells demonstrated ER localization and RESP18 metabolism similar to AtT-20 cells, indicating that the cellular machinery involved in localizing RESP18 is not specific to neuroendocrine cells. The data implicate a novel ER localization mechanism for this neuroendocrine-specific luminal ER resident.