Human Complement Protein C8: The “Hole” Story

Journal of the South Carolina Academy of Science, Dec 1012

By James M. Sodetz, Published on 01/01/12

A PDF file should load here. If you do not see its contents the file may be temporarily unavailable at the journal website or you do not have a PDF plug-in installed and enabled in your browser.

Alternatively, you can download the file locally and open with any standalone PDF reader:

https://scholarcommons.sc.edu/cgi/viewcontent.cgi?article=1027&context=jscas

Human Complement Protein C8: The “Hole” Story

Journal of the South Carolina Academy of Science, [ Human Complement Protein C8: The "Hole" Story James M Sodetz 0 0 Department of Chemistry and Biochemistry, University of South Carolina , Columbia SC 29208 , USA As the recipient of the 2011 SC Governor's Award for Excellence in Scientific Research, I've been asked to describe my research and some of the accomplishments of my laboratory while at the University of South Carolina. My research is focused on understanding the structure and function of the pore-forming proteins of the human"complement system", and in particular complement protein "C8". The complement system is a group of blood proteins that have a key role in immune defense. Much of what is known today about the structure and function of human C8 can be attributed to work performed over many years by my graduate students and postdoctoral fellows. - are all noncovalent. Individually, the five MAC components circulate in blood as hydrophilic proteins, but when combined they form an amphiphilic complex capable of intercalating into cell membranes. The MAC does not degrade membrane lipid but instead produces a disruptive rearrangement that causes osmotic lysis of simple cells such as erythrocytes, initiates intracellular signaling events in nucleated cells, and disrupts the outer membrane of bacteria. Our own cells contain CD59, a membrane-anchored protein that protects us from complement-mediated damage by preventing assembly of a functional MAC. The MAC components interact in a highly specific and sequential manner once C5b is formed. As each intermediate complex is formed, binding specificity changes and is directed towards the next component incorporated. Once associated, the affinity between components is high despite the noncovalent nature of their interactions. Dissociation can only be accomplished by solubilization of the membrane and denaturation of the MAC. The goal of our program has been to identify structural features that specify the order in which these proteins bind, and the mechanism by which they undergo a hydrophilic to amphiphilic transition that enables them to bind to lipid. Our strategy has been to focus on identifying structure-function relationships in one component, namely human C8, and thereby gain insight into how the other MAC components function. Understanding how the MAC is assembled and its function may lead to the development of therapeutic pore-forming analogues that could be used to attack undesirable human cells, e.g. cancer cells. The MAC Family Proteins Human C5b contains two disulfide-linked subunits and is the largest MAC protein (180 kDa). C6, C7 and C9 are single chain proteins of approximately 105 kDa, 92 kDa and 72 kDa, respectively. C8 (151 kDa) is the most complex in that it is composed of three nonidentical, genetically distinct subunits (α = 64 kDa; β = 64 kDa and γ = 22 kDa) ( 7 ). These subunits are arranged as a disulfide-linked C8α-γ heterodimer that is noncovalently associated with C8β. The affinity between C8αγ and C8β is high. Dissociation can only be achieved using denaturing agents or high ionic strength buffers. Our discovery in the 1980's that each C8 subunit was encoded in a different gene was significant because C8 was thought to be a single gene product, i.e. synthesized as a single-chain precursor that was posttranslationally cleaved. This discovery came as we were characterizing human liver cDNA clones to determine the amino acid sequence of C8 ( 8 ). The results established that C8 was composed of three different proteins rather than one. Our efforts to determine the sequences and genomic organization of C8 together with the work of others provided the first evidence of a structural and evolutionary relationship between the MAC proteins. Human C6, C7, C8α, C8β, and C9 are homologous and together form the "MAC family" of proteins. Family members exhibit sequence similarity and have a highly conserved modular organization (Fig. 2) ( 4,9 ). Their genomic structures are also similar with respect to exon length and boundaries. A distinctive feature of each is the presence of cysteine-rich Nand C-terminal modules that are ~40–80 amino acids in length. The modules exhibit sequence similarity to those found in a variety of proteins unrelated to complement. Although not considered a module, the central portion of each protein (~ 40 kDa) is designated MACPF to emphasize its conservation among the MAC proteins and its sequence similarity to perforin, a 70 kDa pore-forming protein released from secretory granules of cytotoxic T lymphocytes. The conserved organization of the MAC proteins initially suggested the modules themselves may mediate proteinprotein interactions during MAC assembly. This was consistent with the view that the MAC can be thought of as a "heteropolymer" formed from structurally similar but distinct monomeric units. Also noteworthy was the fact these modules mediate protein-protein interactions in other systems (...truncated)


This is a preview of a remote PDF: https://scholarcommons.sc.edu/cgi/viewcontent.cgi?article=1027&context=jscas

James M. Sodetz. Human Complement Protein C8: The “Hole” Story, Journal of the South Carolina Academy of Science, 1012, Volume 10, Issue 1,