2 LINEAGES OF MANNOSE-BINDING LECTIN-ASSOCIATED SERINE-PROTEASE (MASP) IN VERTEBRATES

Mannose-binding lectin-associated serine protease (MASP) is a newly id entified member of the serine protease superfamily, MASP is involved i n host defense against: pathogens through a novel system of complement activation, designated the lectin pathway. To elucidate the origin of the lectin pathway and the molecular evolution of MASP, me cloned six MASP cDNAs from live vertebrate species going from mammal to cyclosto me. An alignment of the amino acid sequences deduced from the cDNAs re vealed the presence of two different lineages of the MASP gene. This c lassification was supported by the precise correlation with two types of exon organization for the protease domain. One of the two lineages is unique in that a single exon encodes the protease domain, unlike mo st other serine proteases. All members of this group, termed the AGY t ype, have an AGY codon at the! active site serine. A phylogenetic tree suggests that the AGY type diverged from another lineage, termed the TCN type, before the emergence of primitive vertebrates. Furthermore, the presence of MASP or MASP-like sequences in most vertebrate species suggests that the lectin pathway functions extensively in vertebrates and that its origin is traced back to the invertebrate stage.