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Mitotic spindles were isolated from Chinese hamster ovary (CHO) cells and examined morphologically and biochemically. The isolated spindles were observed to be intact structures containing associated chromosomes and were surrounded by a cage of vimentin-containing filaments. Two-dimensional gel electrophoresis of isolated spindles versus whole cell homogenates indicated that isolated spindles were free from significant cytoplasmic contamination and contained tubulin, actin, vimentin and an 80 X 10(3) Mr quadrapeptide as their major protein constituents. Five calmodulin-binding proteins with molecular weights of 200, 160, 130, 60 and 52 (X 10(3] Mr were identified within isolated spindles. These calmodulin-binding proteins may be involved in regulating microtubule organization and depolymerization during karyokinesis.

Calcium-dependent regulation of cellular processes is mediated by specific intracellular proteins. A newly described set of proteins isolated from chicken gizzard with Mr of 67 X 10(3), 35 X 10(3), 33 X 10(3) and 30 X 10(3) also express a hydrophobic site in the presence of calcium. These proteins are isolated from several other cellular tissues and are termed calcimedins. These proteins differ from calmodulin in isoelectric point, DEAE-cellulose binding characteristics and heat stability. The calcimedins do not activate calmodulin-dependent cyclic nucleotide phosphodiesterase but do activate a hepatic microsomal Ca2+ -ATPase system. Hence, the possibility is opened that calcium regulation of cellular processes is mediated by calcium-binding proteins in addition to calmodulin.

We examined the subcellular localization of calmodulin in several cultured cells (primary thyroid follicular cells, thyroid C-cell tumour cells (TT), kidney cells (PtK2-L23) and peritoneal macrophages) by indirect immunofluorescence using affinity-purified antibody to calmodulin. When cells were fixed with 3% formaldehyde for 15 min prior to lysis with 0.5% Triton X-100, spindle fibres in mitotic cells were fluorescent and a diffuse cytoplasmic localization of calmodulin was observed in resting cells. However, when cells were lysed with 0.5% Triton X-100 for 90s prior to fixation for 30 min with 3% formaldehyde, three effects were observed. One: there was little diffuse cytoplasmic staining. Two: discrete vesicles were stained. Three: spindle fibres in mitotic cells were fluorescent. The stained vesicles were phase-dense and ranged from 0.1 to 0.5 micron in primary thyroid follicular cells but were smaller in PtK2 and TT cells. The thyroid follicular cells retained vesicular staining after exposure to thyrotropin and isobutylmethylxanthine, but the number of labelled vesicles decreased by almost 80%. Phase-dense vesicles were identified as lysosomes or other acidic vesicles by vital staining with Acridine Orange. After differential centrifugation of thyroid homogenates, calmodulin was measured by radioimmunoassay (RIA) and found in both cytosolic (89%) and membrane vesicle fractions (11%). The vesicular calmodulin was not eluted by washing with 5 mM-EGTA. The thyroid fractions were subjected to SDS-polyacrylamide gel electrophoresis and the gels incubated with 125I-labelled calmodulin to reveal calmodulin acceptor proteins (CAPs). The vesicle fraction contained quantitatively major CAPs with Mr of 200,000, 140,000, 89,000, 38,000 and 34,000, and minor CAPs of 60,000 and 50,000. Washing the pellet with 5 mM-EGTA did not reduce the content of CAPs. Thus, calmodulin and CAPs are present both in the cytoplasm and in a membrane vesicle fraction. The lysosomal locale of calmodulin and the effect of thyrotropin on vesicle number suggest a role for calcium in the regulation of lysosome function.