Peptidyl-prolyl isomerization for cytokinetic abscission

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The peptidyl prolyl isomerase cyclophilin A localizes at the centrosome and the midbody and is required for cytokinesis

Cytokinesis is the final stage of cell division, during which physical separation of the two daughter cells is accomplished. It starts with ingression of the plasma membrane mediated by an actomyosin contractile ring leading to the formation of a cleavage furrow. By the end of furrowing, the dividing cells are connected by an intercellular microtubule-loaded bridge containing the midbody. During the final phase of cytokinesis, in a process called abscission, this bridge is severed, and two daughter cells are formed. Cells that have problems at this stage maintain the intercellular bridge for a prolonged period of time, and if the bridge cannot be severed, cells become polyploid. Polyploidy has been shown to promote tumor development.¹

Central players in abscission are the members of the ESCRT (endosomal sorting complex required for transport) machinery, which constrict the intercellular bridge to allow its fission.² These factors are recruited to the site of abscission by a protein named CEP55.³ CEP55’s function, in turn, is regulated by phosphorylation, and the phosphorylation-dependent peptidyl-prolyl isomerase Pin1.⁴ Pin1 is a member of the parvulin family of prolyl isomerases that catalyze cis-trans isomerization of proline amide bonds. Another family of peptidyl-prolyl isomerases are the cyclophilins. Generally, these isomerases control the folding/conformation of proteins.

In a new study, Bannon et al. reported their analysis of the whereabouts and functions of one of the cyclophilins, cyclophilin A (CypA, also called peptidyl-prolyl isomerase A) during cell division.⁵ They demonstrate that CypA is a centrosomal protein during interphase and mitosis, and that it translocates to the midbody during cytokinesis. Although CypA is centrosomal, CypA deficiency does not cause any detectable mitotic abnormalities. Bipolar spindle formation, the spindle assembly/mitotic checkpoint and the time taken from anaphase until furrow ingression in telophase are all comparable in CypA-knockout and control cells, indicating that its function at the centrosome, if any, is at least redundant. However, knockdown or knockout of CypA does cause a defect at a later stage, namely during cytokinesis. More specifically, cells lacking CypA remained connected by a thin cytoplasmic intercellular bridge for a prolonged period of time and displayed a higher number of polyploid cells as compared with control cells. Moreover, CypA-deficient cells exhibit supernumerary centrosomes and decreased proliferation. By using a mutant of CypA (R55A) that has compromised isomerase activity, the authors showed that isomerization activity of CypA is not required for its localization to the intercellular bridge but is essential for successful completion of cytokinesis.

An interesting area for further studies may include the identification of substrates of CypA that control cytokinesis. To this end, database mining for the CypA consensus binding motif FGPXLP could be informative.⁶ In this motif, only the amino acids FGP are very strict. Another, complementary, approach to identify CypA substrates would be to search for interaction partners of CypA that are known to play a role in cytokinesis. Notably, cells that have reduced activity of calcineurin, a heterodimeric protein phosphatase that interacts with CypA and does contain a CypA consensus binding motif, display cytokinesis defects that are similar to those seen in CypA-knockdown cells.^7,8 Moreover, cyclosporine A (CsA), a fungal immunosuppressive compound that can inhibit both calcineurin and CypA, induces a cytokinesis defect.⁸ Interestingly, CypA may stimulate calcineurin activity.⁷ Whether this is dependent on isomerase activity remains an open question. Nevertheless, it is tempting to speculate that CypA may regulate cytokinesis through activation of calcineurin.

Peptidyl-prolyl isomerization plays a crucial role in a plethora of cellular processes, and the results of Bannon et al. together with previously published data suggest that at least two isomerases, CypA and Pin1, regulate cytokinetic abscission.^4,5 Interestingly, in yeast, the loss of ESS1 (yeast Pin1) can be complemented by enhanced expression of CPH1 (CypA), suggesting that Pin1 substrates can be isomerised by CypA⁹. However, it appears that in mammalian cells, they are not acting redundantly, as knockdown/knockout of each individually (Pin1 or CypA) results in a cytokinesis defect.⁴

CypA is overexpressed in multiple cancers, and it has been hypothesized that tumor cells exploit protein isomerization to promote their growth. Indeed, CypA deficiency does affect cell division and thus may be used as a therapeutic target to limit the proliferation of tumor cells.