Purification and assay of a 145-kDa protein (STOP145) with microtubule-stabilizing and motility behavior.

The capacity of microtubules to disassemble in vitro is profoundly affected by a protein factor designated STOP (stable tubule only polypeptide). Here we report the isolation of STOP protein and confirm that its activity is, as predicted, highly substoichiometric to the tubulin in microtubules. The isolation of the 145-kDa STOP (STOP145) protein has been effected from isolated cold-stable microtubules by two column steps: DEAE ion-exchange and a calmodulin affinity column. To confirm the protein's activity we have produced an antibody against STOP145 and have used the antibody to specifically remove the protein and the activity using an antibody-linked affinity column. We conclude that the STOP145 protein accounts for the observed in vitro stabilization of microtubules.