A cDNA clone from the sea lamprey Petromyzon marinus coding for a scavenger receptor Cys-rich (SRCR) domain protein

Our knowledge of the immune system in the early vertebrates, the Agnatha, and the molecules involved in their immune reactions is fragmentary. By serendipity we discovered a cDNA clone in a library made from gut poly(A)+RNA of the sea lamprey, Petromyzon marinus (Pema), that translates into the SREG (SRCR-EGF, see below) protein which resembles cell-membrane proteins of mammalian immune cells. The putative translated product is a type-I integral membrane glycoprotein which contains two scavenger receptor Cys-rich (SRCR) domains flanking five epidermal growth factor (EGF)-like repeats. The two SRCR domains are closely related to CD6 (expressed on human lymphocytes), WC1 (expressed on mammalian CD4-CD8(-)-gamma delta T cells) and M130 (expressed on human macrophages). The Pema-SREG may therefore be involved in intercellular contacts and cell activation or differentiation in the immune system. It is thus a potential marker that can be used to investigate the lamprey immune system.