Architecture and polypeptide composition of HeLa cytoskeletons: Modification of cytoarchitectural polypeptides during mitosis

Abstract

Substrate-attached asynchronous HeLa cells were extracted with Triton X-100 and analysed by electron microscopy and two-dimensional gel electrophoresis. Such Triton cytoskeletons showed actin filament bundles, microtubules, intermediate filaments, and actin networks in the substrate-associated lamellae, and contained around 90 polypeptides (48 basic, 42 acidic; 52% of total actin, 99% of vimentin, 41% of α-actinin and 30% of β-tubulin). Cytoskeletons produced by further extraction in high and low salt buffers (L-H-L) showed only intermediate filaments, the nucleus and residual actin, and contained a total of 19 polypeptides (13 acidic, 6 basic). Of these, 12 corresponded to abundant acidic proteins in the 47,000 to 70,000 M r region as determined by staining with Coomassie blue and labelling with a mixture of 14C-labelled amino acids. Using L-H-L extracted cytoplasts, and employing an actin depolymerising protein from slime moulds, seven abundant acidic IEF ‡ ‡ IEF, isoelectric focusing. polypeptides were shown to be present in these intermediate filament-enriched, substrate-attached cytoplast cytoskeletons. These polypeptides (L-H-L cytoplast polypeptides) corresponded to vimentin (IEF 26, 54,000 M rmr) and six polypeptides (IEF 12, 68,000 M r; IEF 24, 56,000 M r; IEF 31, 50,000 M r; IEF 35, 49,000 M r; IEF 36, 48,500 M r and IEF 46, 43,500 M r) not previously reported as present in cytoskeletons. Peptide analysis showed that these were not related as products of modification or proteolysis. Labelling of mitotic and interphase cells with [ 35S]methionine followed by one-dimensional peptide map analysis showed that IEF 24, 26 (vimentin), 31 and 36 are preferentially modified during mitosis. These modifications correspond to phosphorylations of IEF 26 (vimentin) and 31, and to an unknown type for IEF 24. IEF 36 is phosphorylated in interphase to yield IEF 37, and the latter is further phosphorylated in mitosis. These results suggest that modification of the L-H-L cytoplast polypeptides may be important in the reorganization of cytoskeletal elements that takes place during cell division.