Partial amino acid sequence of human factor D:homology with serine proteases.

Abstract

Human factor D [a trace serum protein playing an essential role in the activation sequence of the alternative complement pathway] purified to homogeneity by a modified procedure was subjected to NH2-terminal amino acid sequence analysis by using a modified automated Beckman sequencer. Of the 1st 57 NH2-terminal amino acids, 48 were identified in a single sequencer run using microgram quantities of factor D. The deduced amino acid sequence represents approximately 25% of the primary structure of factor D. This extended NH2-terminal amino acid sequence of factor D was compared to that of other trypsin-related serine proteases. By visual inspection, strong homologies (33-50% identity) were observed with all the serine proteases included in the comparison. Interestingly, factor D showed a higher degree of homology to serine proteases of pancreatic origin than to those of serum origin.