Domain organization of the extracellular region of CD45.

CD45 is a large, heavily glycosylated, transmembrane protein phosphotyrosine phosphatase found on all nucleated cells of haematopoietic origin. In lymphocytes, the cytoplasmic phosphatase is necessary for efficient signalling through the antigen receptor but in contrast little is known about the interactions of the extracellular region of the molecule. This consists of a mucin-like region, a novel cysteine-containing region and a region containing three putative fibronectin type III domains. To confirm this organization and to identify parts potentially important for function, we have expressed fragments of the extracellular domain of rat CD45 as recombinant soluble proteins. Proteins corresponding to two, three and four domains of CD45 were expressed in secreted forms. Single domains and constructs for proteins with truncations of the predicted domains were not expressed. This is consistent with the proposed structural organization. Determination of the positions of the disulphide bonds in the N-terminal cysteine-containing region and the first fibronectin type III domain identified novel disulphide bonds within the fibronectin type III domain and an unusual inter-domain disulphide linkage. Circular dichroism spectroscopy indicated that this region of rat CD45 has mainly beta-strand secondary structure and no alpha-helical content. These studies support the proposed domain organization of CD45.