Processing in the C-terminal domain of minicollagen XII removes a heparin-binding site

A minicollagen comprising the two C-terminal domains of collagen XII (COL1 and NC1) has been expressed in insect cells and characterized biochemically. An interaction with heparin is demonstrated, which depends on the correct folding of the molecule. After secretion, minicollagen XII is immediately processed to a form lacking heparin binding ability. Processed and unprocessed trimers differ only at the level of the eight or nine C-terminal residues but they reveal different structures as judged from rotary shadowing images. Similar processing is also observed in the medium of transfected human HeLa cells. These data show that a heparin-binding site is present in the C-terminal end of the chicken collagen XII sequence and strongly suggest that proteolytic processing in the NC1 domain can occur in vivo and regulate the interactive properties of collagen XII.

Collagenous Sequence Governs the Trimeric Assembly of Collagen XII

A minicollagen containing the COL1 and NC1 domains of chicken collagen XII has been produced in insect cells. Significant amounts of trimers contain a triple-helical domain in which the cysteines are not involved in inter- but in intrachain bonds. In reducing conditions, providing that the triple-helix is maintained, disulfide exchange between intra- and interchain bonding is observed, suggesting that the triple-helix forms first and that in favorable redox conditions interchain bonding occurs to stabilize the molecule. This hypothesis is verified by in vitro reassociation studies performed in the presence of reducing agents, demonstrating that the formation of interchain disulfide bonds is not a prerequisite to the trimeric association and triple-helical folding of the collagen XII molecule. Shortening the COL1 domain of minicollagen XII to its five C-terminal GXY triplets results in an absence of trimers. This can be explained by the presence of a collagenous domain that is too short to form a stable triple-helix. In contrast, the presence of five additional C-terminal triplets in COL1 allows the formation of triple-helical disulfide-bonded trimers, suggesting that the presence of a triple-helix is essential for the assembly of collagen XII.

Structural analysis of the heparin-binding site of the NC1 domain of collagen XIV by CD and NMR

Type XIV collagen, a fibril-associated collagen with interrupted triple helices (FACIT), interacts with the surrounding extracellular matrix and/or with cells via its binding to glycosaminoglycans (GAGs). To further characterize such interactions in the NC1 domain of chicken collagen XIV, we identified amino acids essential for heparin binding by affinity chromatography analysis after proteolytic digestion of the synthetic peptide NC1(84-116). The 3D structure of this peptide was then obtained using circular dichroism and NMR. The NC1(84-116) peptide appeared poorly structured in water, but the stabilization of its conformation by the interaction with hydrophobic surfaces or by using cosolvents (TFE, SDS) revealed a high propensity to adopt an alpha-helical folding. A 3D structure model of NC1(84-116), calculated from NMR data recorded in a TFE/water mixture, showed that the NC1-heparin binding site forms a amphipathic alpha-helix exhibiting a twisted basic groove. It is structurally similar to the consensus spatial alpha-helix model of heparin-binding [Margalit et al. (1993) J. Biol. Chem. 268, 19228-19231], except that the GAG binding domain of NC1 may be extended over 18 residues, that is, the NC1(94-111) segment. In addition, the formation of a hydrophobic groove upon helix formation suggests the contribution of additional sequences to ensure the stability of the GAG-binding domain. Overall the NC1(84-116) model exhibits a nativelike conformation which presents suitably oriented residues for the interaction with a specific GAG.

Identification and characterization of a heparin binding site within the NC1 domain of chicken collagen XIV

Collagen XIV is known to bind to the dermatan sulfate chain of decorin and to the heparan sulfate chain of perlecan. To study its possible interaction with glycosaminoglycans, the NC1 domain of chicken collagen XIV was overproduced in E. coli. Purified NC1*(6-119)* appears poorly organized (the asterisks indicate the presence of extension sequences), but V8-protease generated fragments containing the 84-108 basic sequence tend to fold into alpha-helix. These fragments interact specifically with heparin, which induces an alpha-helical fold with a maximum effect for equimolar heparin/peptide ratio. These data demonstrate the existence of a glycosaminoglycan binding site in NC1.

Lysyl oxidase-like protein localizes to sites of de novo fibrinogenesis in fibrosis and in the early stromal reaction of ductal breast carcinomas

Lysyl oxidase (LO) initiates the first step in the crosslinking of collagens and elastin and has also been shown to function as a tumor suppressor. The purpose of the present work was to determine whether the products of a newly described LO-like gene (LOXL) that encodes a close homolog of LO, the LO-like (LOL) protein, is associated with extracellular matrix remodeling during fibrotic disorders. Specific antibody against LOL identified proteins of approximately 30, 42, 52 and 68 kd in various cells and in bovine aorta. These proteins were immunochemically distinct from the recombinant LO expressed by fibroblasts and from the bovine aorta LO. The LO gene (LOX) and LOXL were transiently up-regulated at early stages of liver granuloma development in Schistosoma mansoni-infected mice, although the peak of LOL mRNA synthesis preceded that of LO. LOL protein and LO were colocalized at sites of fibrogenesis in human lung fibrosis and in the stromal reaction of bronchiolo-alveolar carcinomas and of in situ ductal breast tumors. In conclusion, the LOL protein was identified as a secreted protein and localized in the extracellular matrix in active fibrotic diseases and in the early stromal reaction of breast cancer.

Degradation of the COL1 domain of type XIV collagen by 92-kDa gelatinase

Type XIV collagen is a newly described member of the fibril-associated collagens with interrupted triple helices (FACITs). Expression of this collagen has been localized to various embryonic tissues, suggesting that it has a functional role in development. All FACITs thus far described (types IX, XII, XIV, and XVI) contain a highly homologous carboxyl-terminal triple helical domain designated COL1. We have studied the capacity of various matrix metalloproteinases (interstitial collagenase, stromelysin, matrilysin, and 92-kDa gelatinase) to degrade the COL1 domain of collagen XIV. We found that only 92-kDa gelatinase cleaves COL1. Furthermore, digestion of whole native collagen XIV by the 92-kDa gelatinase indicates that this enzyme specifically attacks the carboxyl-terminal triple helix-containing region of the molecule. COL1 is cleaved by 92-kDa gelatinase at 30 degrees C, a full 5-6 degrees C below the melting temperature (Tm) of this domain; native collagen XIV is also degraded at 30 degrees C. In comparison to interstitial collagenase degradation of its physiologic native type I collagen substrate, the 92-kDa enzyme cleaved COL1 (XIV) with comparable catalytic efficacy. Interestingly, following thermal denaturation of the COL1 fragment, its susceptibility to 92-kDa gelatinase increases, but only to a degree that leaves it several orders of magnitude less sensitive to degradation than denatured collagens I and III. These data indicate that native COL1 and collagen XIV are readily and specifically cleaved by 92-kDa gelatinase. They also suggest a role for 92-kDa gelatinase activity in the structural tissue remodeling of the developing embryo.

Binding of collagen XIV with the dermatan sulfate side chain of decorin

As an approach to elucidate the role of collagen XIV, which is still unclear, molecules exhibiting affinity for this collagen have been sought in connective tissue. Extracts from fetal bovine tendon were resolved by gel electrophoresis and electrophoretically transferred to nitrocellulose. The blot was overlaid with native collagen XIV and the collagen XIV-binding molecules revealed by immunodecoration with a monoclonal antitype XIV collagen antibody. This experimental approach allowed us to reveal in tendon extracts a diffuse band, with an apparent molecular mass of approximately 100 kDa, that binds collagen XIV. This molecule was also found associated with the fractions containing partially purified type XIV collagen. This 100-kDa molecule was sensitive to chondroitinase ABC and, after chondroitinase digestion, yielded a core protein of about 48 kDa. N-terminal sequence analysis of the proteoglycan after blotting allowed us to identify it as decorin. By solid phase assays we have studied this newly described association between decorin and type XIV collagen and shown that it is a saturable process. In addition, preliminary determination of the domains of the two molecules involved in the association has been performed. The possible role of these interactions is discussed.

Purification and characterization of native type XIV collagen

A new molecule, type XIV collagen, with domains homologous to type IX and XII collagens has been recently discovered in pepsin extracts of fetal bovine tissues (Dublet, B., and van der Rest, M. (1991) J. Biol. Chem. 266, 6853-6858). In the present study, we describe the purification and the characterization of the intact native form of this newly discovered collagen. By using only two chromatographic steps we were able to obtain pure type XIV collagen. Furthermore, minor modifications of the protocol allowed us to perform the simultaneous large scale purification of type XII and type XIV collagens from the same tissue. Intact type XIV collagen migrates on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) as two bands of 220 and 290 kDa (reducing conditions). After collagenase treatment, a single band of 190 kDa is observed, which represents the large non-collagenous domain of the molecule (NC3). Rotary shadowing electron micrographs of intact type XIV collagen show a cross-shaped structure formed by a thin tail attached through a central globule to three identical "fingers." These properties are similar to those previously described for intact chicken type XII collagen (Dublet, B., Oh, S., Sugrue, S. P., Gordon, M. K., Gerecke, D. R., Olsen, B. R., and van der Rest, M. (1989) J. Biol. Chem. 264, 13150-13156), but the two molecules are different gene products and have charge and glycosylation differences. Finally, we show that the three chains of purified type XIV collagen have an apparent molecular mass of approximately 220 kDa and are not cross-linked to each other by bonds other than disulfide bridges. The same observation was made for type XII collagen. In both cases, the 290-kDa migrating band in SDS-PAGE is due to incomplete denaturation in electrophoresis sample buffer in the absence of urea.

Do synapsin I and microtubule-associated proteins bind to a common site on polymerized tubulin?

Synapsin I plays an important role in the regulation of neurotransmitter release, since it binds to synaptic vesicles and to the cytoskeleton, and it bundles F-actin and microtubules. We have previously shown by tryptic digestion of synapsin I that a 44 kDa fragment contains a binding site for polymerized tubulin. In the present experiments, we test whether synapsin I and microtubule-associated proteins (MAPs) have the same or a different binding site on tubulin molecules. Our results show that heat stable MAPs do not compete with synapsin I for binding to taxol tubulin. In addition, subtilisin digestion of tubulin, which suppresses MAPs binding, does not abolish synapsin I cosedimentation with taxol tubulin. Thus, our results strongly suggest that synapsin I (as reported for kinesin) does not bind to the 4 kDa subtilisin digested C-terminal part of the tubulin molecule.

Limited proteolysis of synapsin I. Identification of the region of the molecule responsible for its association with microtubules

Synapsin I is a highly asymmetric neuronal structural phosphoprotein implicated in the regulation of neurotransmitter release probably by the multiple interactions it can contract with membranous and cytoskeletal elements of the neuronal cell. In order to locate the region(s) of synapsin I responsible for its association with microtubules, we have first studied synapsin I limited digestion by trypsin. The resulting polypeptides were localized in the synapsin I molecule by using three different criteria: their kinetics of appearance, their collagenase sensitivity, and the presence of the synapsin phosphorylation site 1 (cyclic AMP dependent). Synapsin I digestion kinetics are not affected by phosphorylation at this site. Analysis of the ability of various synapsin I tryptic fragments in mixture to cosediment with microtubules shows that a 44-kDa fragment corresponding to the NH2-terminal hydrophobic head of the molecule contains a binding site for polymerized tubulin. This fragment competes with native synapsin I for binding on microtubules. None of the polypeptides belonging to the tail region of synapsin I (COOH-terminal half of the molecule) were found to cosediment with microtubules.

Detection by chemical cross-linking of bovine brain synapsin I self-association

Synapsin I is believed to play an important role in the regulation of neurotransmitter release, since it is able to bind to synaptic vesicles, to the cytoskeleton and to membrane proteins; in addition, it bundles F-actin and microtubules. These properties, which are controlled by phosphorylation, could be explained if synapsin has different and multiple binding sites or if synapsin I is able to form polymers by self-association. In this study we present experimental evidence that synapsin I at low concentration forms self-associated dimers, as revealed after mild treatments with cross-linking agents. We have especially studied here the effects of copper/o-phenanthroline, a zero-length cross-linking agent which forms covalent links by oxidative formation of S-S bridges between adjacent cysteines. The time course and concentration-dependence of synapsin-dimer formation are studied; interestingly, these experiments could suggest a different behaviour of the two polypeptides. Limited proteolysis of phosphorylated synapsin I by V8 protease, alpha-chymotrypsin or collagenase, performed on the isolated dimer and monomer, allows us to localize tentatively in the central hydrophobic core of the molecule the cysteine residues the oxidation of which by copper/o-phenanthroline gives rise to synapsin dimers.

Modified properties of hexokinase from heart mitochondria prepared using proteolytic enzyme

Isolation of muscle mitochondria is made easier by using proteolytic treatment of the tissue before homogenization. Normally, the proteolytic enzyme is discarded with the supernatant of the first centrifugation. However, our results show that a fraction of enzyme activity remains associated with mitochondria. As shown in experiments described in this paper, mitochondrial hexokinase from tissue treated or not with the proteolytic enzyme exhibits similar properties except that the solubilized enzyme from protease treated tissue is no longer able to rebind to mitochondrial membrane. This modification of the binding ability of the enzyme results from a partial hydrolysis of hexokinase during solubilization experiments by the proteolytic enzyme. Since, as pointed out here, proteolytic enzyme can remain associated with mitochondria, [either absorbed on mitochondrial membrane or included in the mitochondrial pellet] its use for the isolation of muscle mitochondria should be avoided.

Rabbit heart mitochondrial hexokinase: solubilization and general properties

In rabbit heart, results show that two isoenzymes of hexokinase (HK) are present. The enzymatic activity associated with mitochondria consists of only one isoenzyme; according to its electrophoretic mobility and its apparent Km for glucose (0.065 mM), it has been identified as type I isoenzyme. The bound HK I exhibits a lower apparent Km for ATPMg than the solubilized enzyme, whereas the apparent Km for glucose is the same for bound and solubilized HK. Detailed studies have been performed to investigate the interactions which take place between the enzyme and the mitochondrial membrane. Neutral salts efficiently solubilize the bound enzyme. Digitonin induces only a partial release of the enzyme bound to mitochondria; this result could be explained by the existence of contacts between the outer and the inner mitochondrial membranes [C. R. Hackenbrock (1968) Proc. Natl. Acad. Sci. USA 61, 598-605]. Furthermore, low concentrations (0.1 mM) of glucose 6-phosphate (G6P) or ATP4- specifically solubilize hexokinase. The solubilizing effect of G6P and ATP4-, which are potent inhibitors of the enzyme, can be prevented by incubation of mitochondria with Pi or Mg2+. In addition, enzyme solubilization by G6P can be reversed by Mg2+ only when the proteolytic treatment of the heart homogenate is omitted during the course of the isolation of mitochondria. These results concerning the interaction of rabbit heart hexokinase with the outer mitochondrial membrane agree with the schematic model proposed by Wilson [(1982) Biophys. J. 37, 18-19] for the brain enzyme. This model involves the existence of two kinds of interactions between HK and mitochondria; a very specific one with the hexokinase-binding protein of the outer mitochondrial membrane, which is suppressed by glucose 6-phosphate, and a less specific, cation-mediated one.