Genome and expressed sequence tag information of Xenopus tropicalis suggested 
that short-consensus repeat (SCR)-containing proteins are encoded by three genes 
that are mapped within a 300-kb downstream of PFKFB2, which is a marker gene for 
the regulator of complement activation (RCA) loci in human and chicken. Based on 
this observation, we cloned the three cDNAs of these proteins using 3'- or 
5'-RACE technique. Since their primary structures and locations of the proximity 
to the PFKFB2 locus, we named them amphibian RCA protein (ARC) 1, 2, and 3. 
Expression in human HEK293 or CHO cells suggested that ARC1 is a soluble protein 
of Mr approximately 67 kDa, ARC2 is a membrane protein with Mr 44 kDa, and ARC3 
a secretary protein with a putative transmembrane region. They were 
N-glycosylated during maturation. In human and chicken RCA clusters, the order 
in which genes for soluble, GPI-anchored, and membrane forms of SCR proteins are 
arranged is from the distant to proximity to the PFKFB2 gene. However, the 
amphibian ARC1, 2, and 3 resembled one another and did not reflect the same 
order found in human and chicken RCA genes. This may be due to self-duplication 
of ARCs to form a family, and it evolved after the amphibia separated from the 
ancestor of the amniotes, which possessed soluble, GPI-anchored, and membrane 
forms of SCR protein members. Taken together, frog possesses a RCA locus, but 
the constitution of the ARC proteins differs from that of the amniotes with a 
unique self-resemblance.