Transmembrane Regulation of Ectodomain Shedding

描述（由申请人提供）：胞外结构域脱落，即对整合膜蛋白进行蛋白水解裂解以从宿主细胞释放胞外结构域，影响多种生物学重要蛋白质，包括生长因子前体、细胞因子受体、淀粉样蛋白前体蛋白和细胞粘附受体以及蛋白聚糖。因此胞外区脱落功能障碍常导致各种疾病的发生。本项目的长期目标是阐明跨细胞膜脱落调控的分子和结构基础。细胞内蛋白可以通过与脱落蛋白底物的胞质结构域直接相互作用来调节脱落。最好的表征实例是钙调素（CaM）与L-选择素结合以抑制L-选择素的脱落。调节机制的一个线索是有据可查但无法解释的观察结果，即L-选择素中的近膜脱落切割位点与细胞膜之间的距离，而不是切割位点处的序列，对脱落活性至关重要。缩短距离可消除脱落。L-选择素胞质结构域的膜近端区域与CaM相互作用，但它只有12个残基长，比典型的CaM结合序列短得多。我们最近对CaM与L-选择素衍生肽相互作用的研究表明，L-选择素中的CaM结合区也可能包括L-选择素跨膜结构域的一部分。我们假设钙调素与L-选择素的相互作用影响其在膜中的跨膜结构域，这反过来又改变了膜另一侧的脱落切割位点的构象和/或可及性。L-选择素跨膜结构域的一部分可以在CaM缔合时分配到CaM中，从而将整个TM结构域和脱落切割位点向细胞质移动，缩短脱落切割位点与膜之间的距离，并有效地抑制L-选择素的脱落。在具体目标1中，将进一步表征CaM与来自L-选择素的水溶性肽相互作用的能量和结构基础。在具体目标2中，钙调素与L-选择素片段在膜模拟环境中的相互作用将通过NMR和荧光光谱进行表征。该片段含有脱落切割位点、L-选择素的跨膜和胞质结构域。重点将是检测钙调素协会诱导的L-选择素跨膜结构域的任何构象和/或位置的变化。在特定目标3中，将表征由CaM缔合诱导的L-选择素片段中脱落切割位点的变化，包括脱落切割位点与膜双层之间距离的任何变化。对CaM与L-选择素肽的复合物的仔细研究将有助于阐明CaM调节L-选择素脱落的潜在机制，并提供对一般脱落调节机制的见解。公共卫生相关性：与公共卫生的相关性：胞外结构域脱落影响多种生物学重要蛋白质。它的功能失常往往导致疾病。阐明钙调素调节L-选择素脱落的机制将有助于更好地理解脱落调节。我们的发现可能会导致新的治疗策略。

DESCRIPTION (provided by applicant): Ectodomain shedding, the proteolytic cleavage of an integral membrane protein to release the extracellular domain from the host cell, affects a variety of biologically important proteins including growth factor precursors, cytokine receptors, amyloid precursor proteins and cell adhesion receptors, and proteoglycans. Therefore malfunction of ectodomain shedding often leads to various diseases. The long- term objective of this project is to elucidate the molecular and structural basis for shedding regulation across the cell membrane. Intracellular proteins can regulate shedding by interacting directly with the cytoplasmic domain of the shedding protein substrate. The best characterized example is the calmodulin (CaM) association with L-selectin to inhibit shedding of L-selectin. One clue to the regulation mechanism is the well documented but unexplained observation that the distance between the membrane-proximal shedding cleavage site in L-selectin and the cell membrane, rather than the sequence at the cleavage site, is critical to shedding activity. Shortening the distance abolishes shedding. The membrane-proximal region of the L-selectin cytoplasmic domain interacts with CaM, but it is only 12 residues long, much shorter than a typical CaM-binding sequence. Our recent study on the interaction of CaM with L-selectin-derived peptides suggested that the CaM-binding region in L-selectin may also include a portion of the L-selectin transmembrane domain. We hypothesize that CaM interaction with L-selectin affects its transmembrane domain in the membrane, which in turn changes the conformation and/or accessibility of the shedding cleavage site on the other side of the membrane. A portion of the L-selectin transmembrane domain may, upon CaM association, partition into CaM, thereby moving the entire TM domain and shedding cleavage site toward the cytoplasm and shortening the distance between the shedding cleavage site and the membrane, and effectively inhibit shedding of L-selectin. In Specific Aim 1, the energetic and structural basis for the interaction of CaM with water-soluble peptides derived from L-selectin will be further characterized. In Specific Aim 2, the interaction of CaM with a L-selectin fragment in membrane-mimicking environments will be characterized with NMR and fluorescence spectroscopy. The fragment contains the shedding cleavage site, the transmembrane and cytoplasmic domains of L-selectin. The focus will be to detect any conformational and/or positional changes in the L-selectin transmembrane domain induced by CaM association. In Specific Aim 3, changes at the shedding cleavage site in the L-selectin fragment induced by CaM association, including any changes in the distance between the shedding cleavage site and the membrane bilayer, will be characterized. A careful study of the complexes of CaM with the L- selectin peptides will help to elucidate the mechanism underlying CaM regulation of L-selectin shedding, and provide insights into shedding regulation mechanisms in general. PUBLIC HEALTH RELEVANCE: Relevance to public health: Ectodomain shedding affects a variety of biologically important proteins. It malfunction often leads to diseases. Elucidating the mechanism underlying calmodulin regulation of L- selectin shedding will contribute to a better understanding of shedding regulation. Our findings may lead to novel therapeutic strategies.