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Recent results [1] on the structure and function of a receptor protein tyrosine phosphatase, RTPTm, will be presented. In addition to their intercellular catalytic domains which bear the phosphatase activity, the RPTPs are cell surface receptor type molecules and in many cases have large extracellular regions. CD45 is one such example. What role can these extracellular regions play in function? For RTPTm the extracellular region is known to mediate homophilic adhesion. Sequence analysis indicates that it comprises of six domains: an N terminal MAM (meprin/A5/m), one immunoglobulin-like domain and four fibronectin type III (FN) repeats. We have determined the crystal structure of the entire extracellular region for RTPTm in the form of a functional adhesion dimer. The physical characteristics and dimensions of the adhesion dimer suggest a mechanism by which the location of this phosphatase can be influenced …