NONNEURONAL ISOFORMS OF STOP PROTEIN ARE RESPONSIBLE FOR MICROTUBULE COLD STABILITY IN MAMMALIAN FIBROBLASTS

A number of cycling mammalian cells, such as NIH 3T3, contain abundant subsets of cold-stable microtubules, The origin of such microtubule s tabilization in nonneuronal cells is unknown, We have previously descr ibed a neuronal protein, stable tubule-only polypeptide (STOP), that b inds to microtubules and induces cold stability. We find that NIH 3T3 fibroblasts contain a major 42-kDa isoform of STOP (fibroblastic STOP, F-STOP), F-STOP contains the central repeats characteristic of brain STOP but shows extensive deletions of N- and C-terminal protein domain s that are present in brain STOP. These deletions arise from differenc es in STOP RNA splicing, Despite such deletions, F-STOP has full micro tubule stabilizing activity. F-STOP accumulates on cold-stable microtu bules of interphase arrays and is present on stable microtubules withi n the mitotic spindle of NIH 3T3 cells, STOP inhibition by microinject ion of affinity-purified STOP central repeat antibodies into NIH 3T3 c ells abolishes both interphase and spindle microtubule cold stability. Similar results were obtained with Rat2 cells. These results show tha t STOP proteins have nonneuronal isoforms that are responsible for the microtubule cold stability observed in mammalian fibroblasts.