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 previously shown that cell spreading is uncoupled from adhesion when unglycosylated laminin is used as a substratum; spreading was restored by a Pronase digest of laminin which became inactive when it was specifically depleted of its mannoside peptides; spreading was also specifically restored by mannosides such as mannan, Man9, and Man6, but not Man3. The effector mannosides bind to a cell surface receptor, previously shown by direct and indirect methods. We have now identified the receptor as cell surface calreticulin by isolating it via mannan affinity chromatography 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 calreticulin from whole cell lysates successfully competes with cell surface calreticulin and prevents cell spreading. The composite data implicate cell surface calreticulin as a putative lectin that must be occupied to initiate spreading of laminin-adherent B16 cells.