We studied the characteristics of cytoplasmic microtubule reassembly from endogenous tubulin pools in situ using a Brij 58-lysed 3T3 cell system. Cells that were pretreated in vivo with colcemid retain endogenous tubulin in the depolymerized state after lysis. When lysed cells were removed from colcemid block and incubated in GTP-PIPES reassembly buffer at pH 6.9, microtubules repolymerized randomly throughout the cytoplasm, appeared to be free-ended and were generally not associated with the centrosomes. However, tubulin could be induced to polymerize in an organized manner from the centrosomes by increasing the pH to 7.6 in the presence of ATP and cAMP. Microtubules polymerized in ATP had significantly longer lengths than those assembled in GTP or UTP. When cells not treated with colcemid were lysed, the integrity of the cytoplasmic microtubule complex (CMTC) was maintained during subsequent incubation in reassembly buffer. However, in contrast to unlysed, living cells, microtubules of lysed cells were stable to colchicine. A significant fraction of the CMTC was stable to cold-induced disassembly whereas microtubules reassembled after lysis were extremely cold-sensitive. When cells not treated with colcemid were lysed and incubated in millimolar Ca++, microtubules depolymerized from their distal ends and a much reduced CMTC was observed. Ca++ reversal with EGTA rapidly resulted in a reformation of the CMTC apparently by elongation of Ca++ resistant microtubules.

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