Ste20-related Protein Kinase LOSK (SLK) Controls Microtubule Radial Array in Interphase

Abstract

Interphase microtubules are organized into a radial array with centrosome in the center. This organization is a subject of cellular regulation that can be driven by protein phosphorylation. Only few protein kinases that regulate microtubule array in interphase cells have been described. Ste20-like protein kinase LOSK (SLK) was identified as a microtubule and centrosome-associated protein. In this study we have shown that the inhibition of LOSK activity by dominant-negative mutant K63R-⌬T or by LOSK depletion with RNAi leads to unfocused microtubule arrangement. Microtubule disorganization is prominent in Vero, CV-1, and CHO-K1 cells but less distinct in HeLa cells. The effect is a result neither of microtubule stabilization nor of centrosome disruption. In cells with suppressed LOSK activity centrosomes are unable to anchor or to cap microtubules, though they keep nucleating microtubules. These centrosomes are depleted of dynactin. Vero cells overexpressing K63R-⌬T have normal dynactin "comets" at microtubule ends and unaltered morphology of Golgi complex but are unable to polarize it at the wound edge. We conclude that protein kinase LOSK is required for radial microtubule organization and for the proper localization of Golgi complex in various cell types. INTRODUCTIONThe radial array of microtubules is typical for many mammalian cells. It organizes bidirectional organelle transport in the cytoplasm in the endocytotic and exocytotic direction. It is also required for the regulation of interaction of microtubule plus ends with cell periphery. Both functions are important for cell polarization, movement, and signal transduction (Hyman and Karsenti, 1996;Dujardin et al., 2003;Morrison, 2007). The degree of radiality of microtubules varies in different types of cells. For instance, in fish melanophores the system of microtubules is perfectly radial (Schliwa et al., 1978), whereas in myotubes it is unclear (Tassin et al., 1985;Musa et al., 2003). Moreover, among fibroblast-like cultured mammalian cells some (green monkey kidney Vero or Chinese hamster ovary CHO-K1) possess a distinct radial microtubule array (Bre et al., 1987), whereas others (human cervical carcinoma HeLa or mouse fibroblasts NIH 3T3) have rather chaotic microtubule arrangements (Bulinski and Borisy, 1980). Regardless of the tissue origin of cells, microtubules become more radial when cultured cells are sparse and less radial in confluent cultures. It seems that the status of microtubule organization is regulated by signal transduction pathways and depends on cell differentiation.The focused microtubule arrays can also be formed in acentrosomal cell fragments as a result of interactions between microtubules and membrane vesicles, covered with motor proteins (Rodionov and Borisy, 1997;Malikov et al., 2005). In acentrosomal fragments of fish melanocytes microtubules are chaotic when pigment granules are dispersed. After the addition of adrenaline or other activation of melanosome aggregation microtubules assemble in a radial array. Thus, in this case...

The Ste20-like kinase SLK is required for ErbB2-driven breast cancer cell motility

“…A role for SLK in Plk1 activation during the cell cycle has been recently suggested (Ellinger-Ziegelbauer et al, 2000). Supporting this, SLK was found to be important for microtubule radial array orientation (Burakov et al, 2008).…”

SLK-mediated phosphorylation of paxillin is required for focal adhesion turnover and cell migration

“…Interestingly, SLK demonstrates a dramatic decrease in kinase activity under these adhesion-stabilizing conditions, suggesting a role for activated SLK in the regulation of dynamic focal adhesions [69]. In contrast to its ability to remodel actin, overexpression of SLK does not affect the organization of the microtubule network: SLK has been shown to regulate both microtubule radial array orientation and golgi polarization during interphase, as well as cell cycle progression through G2/M, without affecting microtubule stability [78,80,81].…”

Ste20-like kinase SLK, at the crossroads

“…39,40 Further study revealed that SLK was required for radial microtubule organization during interphase as depletion of the kinase resulted in the inability of centrosomes to anchor or cap microtubules. 21 Interestingly, overexpression of SLK induced cell cycle re-entry of Xenopus oocytes. 39 These observations offer a possible mechanism by which overexpression of SLK leads to apoptosis.…”

“…The carboxy-terminus of SLK shares a high degree of identity with AT1-46 at amino acids 788-936 (63%) and 957-1171 (71%) as well as with LOK (56%) between amino acids 867 and 1178. 14,17,21,22 This C-terminal coil motif was termed the ATH domain, for AT1-46 homology domain. 14,23 A putative caspase 3 consensus cleavage site (DXXD) is also contained within the central domain at amino acid residue 436 of the mouse sequence and is loosely conserved in several other species.…”

“…It was reported that complete deletion (SLK ) or addition of the c-terminus has no effect of SLK kinase activity. 21,22 Another study has shown that deletion of the ATH increased SLK kinase activity. 17 One explanation is the fact that different cell lines (COS-1 and 293) and kinase assay protocols were used in the different studies.…”