Domain organization of the extracellular region of CD45

CD45 is a large, heavily glycosylated, transmembrane proteinphosphotyrosine phosphatase found on all nucleated cells ofhaematopoietic origin. In lymphocytes, the cytoplasmic phosphataseis necessary for efficient signalling through the antigen receptorbut in contrast little is known about the interactions of theextracellular region of the molecule. This consists of a mucin-likeregion, a novel cysteine-containing region and a region containingthree putative fibronectin type III domains. To confirm thisorganization and to identify parts potentially important forfunction, we have expressed fragments of the extracellular domainof rat CD45 as recombinant soluble proteins. Proteins correspondingto two, three and four domains of CD45 were expressed in secretedforms. Single domains and constructs for proteins with truncationsof the predicted domains were not expressed. This is consistentwith the proposed structural organization. Determination ofthe positions of the disulphide bonds in the N-terminal cysteine-containingregion and the first fibronectin type III domain identifiednovel disulphide bonds within the fibronectin type III domainand an unusual inter-domain disulphide linkage. Circular dichroismspectroscopy indicated that this region of rat CD45 has mainlyß-strand secondary structure and no -helical content.These studies support the proposed domain organization of CD45.