the data confirmed many points previously eluci- 
dated, the finer resolution of YAC mapping allowed 
the discovery and/or localization of partial gene du- 
plications, the determination of gene orientations, 
and the measurement of gaps between known genes. 
Nine overlapping YACs that encompass a genomic 
region of 800 kb, encoding four RCA genes and 
three gene-like elements, were identified. 
The encoded genes and two of the gene-like ele- 
ments shared the same orientation and were ordered 
(5' to 3') DAF, CR2, CRl, MCP-like, CRMike, and 
MCP. A C4bp-like region lies upstream from DAF. 
MCP-like, a new genetic element, was discovered 
and found to be homologous to the 5' portion of the 
MCP gene. Two large gaps of 85 kb (between CR2 
and DAF) and 1 1 0 kb (between DAF and the C4bp- 
like element) could carry additional RCA genes. 
The arrangement of CRl , MCP-like, CRl -like, and 
MCP, in that order, strongly suggests that this region 
was generated by a single duplication of neighbor- 
ing CRl/CRl-like and MCP/MCP-like forerunners. 
The RCA YACs will now serve as a convenient DNA 
source for the subcloning and further characteriza- 
tion of this region. 
C3b/C4b receptor, or complement receptor type 
1 ( CRl, CD35J. CRl is expressed on most periph- 
eral blood cells, including erythrocytes, where it is 
a critical player in the processing of immune com- 
plexes. The Knops, McCoy, Swain-Langley, and York 
antigens were identified as being on CRl . The rela- 
tionship between CRl expression and the reactivity 
of the CRl -related blood group antigens with their 
specific antibodies was assessed. 
Red blood cells (RBCs) from donors of selected 
phenotypes were tested by hemagglutination, using 
two monoclonal antibodies to CRl and the antisera 
to the specific blood groups. Monoclonal antibodies 
(mAbs) 3D9 and Ell required ~250 and ~400 
CRl/RBC to obtain a positive reaction. Agglutina- 
tion of antigen-positive cells by human polyclonal 
antisera was also related to the CRl/RBC. Thus cells 
expressing 20-100 CRl /RBC were negative and in- 
cluded the previously designated null phenotypes 
for this collection, cells expressing 100-150 were 
weak or negative, and those expressing >200 were 
usually positive. These data provide an explanation 
for previously puzzling serologic characteristics of 
the CRl -related blood group antigen system. 
Membrane cof actor protein (MCP or CD46). 
MCP is a C regulatory protein that is widely ex- 
pressed on human cells and cell lines. It binds C3b 
and C4b and functions as a cofactor for their degra- 
dation. Because of this, MCP is postulated to protect 
autologous cells from C-mediated injury. 
Human MCP was shown to protect transfected ro- 
dent cells from human C-mediated lysis. Further- 
more, MCP inhibited C3b deposition in a dose- 
dependent fashion and inhibited lysis of the mouse 
cells expressing it. MCP did not inhibit lysis on by- 
stander cells. These results demonstrate the protec- 
tive role of MCP for the cell on which it is 
expressed. 
The predominant structural motif of MCP is the 
four short consensus repeats (SCRs) . These domains 
are responsible for ligand binding in other C regula- 
tory proteins. SCR-deletion mutants were con- 
structed to determine which of the four SCRs of 
MCP contribute to ligand-binding and cofactor activ- 
ity. Analysis of the deletion mutants indicated that 
the third and fourth SCRs were important for both 
ligand-binding and cofactor activity of C3b and 
C4b. In addition, the same SCRs were required for 
binding of an mAb that inhibits MCP's function. 
The deleted mutant of SCR-2 bound but lacked 
cofactor activity for C3b. It did not bind or possess 
cofactor activity for C4b. Deletion of the first 
(amino-terminal) SCR had a minimal effect on C3b- 
binding and cofactor activity but reduced the effi- 
ciency of C4b binding. The results identify the SCRs 
of MCP that contribute to ligand-binding and cofac- 
tor activity. The data also suggest the presence of 
distinguishable C3b- and C4b-binding sites and pro- 
vide evidence that C3b binding is not always suffi- 
cient for cofactor activity. 
An MCP-like molecule on the inner acrosomal 
membrane of human spermatozoa has been charac- 
terized. Three mAbs and a rabbit polyclonal anti- 
body against MCP recognized the sperm protein. On 
SDS-PAGE (sodium dodecyl sulfate-polyacrylamide 
gel electrophoresis), sperm MCP migrated as a sin- 
gle band with a molecular mass of 38 and 44 kDa 
under nonreducing and reducing conditions, re- 
spectively. The molecular mass is 10-20 kDa less 
than the forms of MCP expressed on other cells. 
In contrast to the MCP of other cells, the electro- 
phoretic pattern, by one- and two-dimensional gel 
analysis, and the isoelectric point profile (4.5 to 
5 .0) of the sperm protein were similar among multi- 
ple individuals. Furthermore, digestion with endo- 
glycosidases did not alter either the molecular mass 
or the isoelectric point of the protein, suggesting 
that it is a poorly or nonglycosylated form of MCP. 
The solubilized sperm protein bound C3b and pos- 
sessed cofactor activity for factor I-mediated cleav- 
age of C3b. An mAb that blocks the regulatory func- 
tion of MCP inhibited the cofactor activity of the 
sperm lysate. 
Thus the sperm protein is an antigenic and func- 
tional homologue of MCP but has the distinct struc- 
tural feature of a lower molecular mass secondary to 
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