CELL-SURFACE CALRETICULIN IS A PUTATIVE MANNOSIDE LECTIN WHICH TRIGGERS MOUSE MELANOMA CELL SPREADING

B16 mouse melanoma cells adhere to and spread on laminin. We have prev iously shown that cell spreading is uncoupled from adhesion when ungly cosylated laminin is used as a substratum; spreading was restored by a Pronase digest of laminin which became inactive when it was specifica lly depleted of its mannoside peptides; spreading was also specificall y restored by mannosides such as mannan, Man9, and Man6, but not Man3. The effector mannosides bind to a cell surface receptor, previously s hown by direct and indirect methods, We have now identified the recept or as cell surface calreticulin by isolating it via mannan affinity ch romatography and showing its sequence identity with mouse calreticulin , Anti-calreticulin antibodies confirm this identity, decorate the B16 cell surface, and block cell spreading, Purified B16 cell calreticuli n from whole cell lysates successfully competes with cell surface calr eticulin and prevents cell spreading. The composite data implicate cel l surface calreticulin as a putative lectin that must be occupied to i nitiate spreading of laminin-adherent B16 cells.