A Kinesin Mutant with an Atypical Bipolar Spindle Undergoes Normal Mitosis

Abstract

Motor proteins have been implicated in various aspects of mitosis, including spindle assembly and chromosome segregation. Here, we show that acentrosomal Arabidopsis cells that are mutant for the kinesin, ATK1, lack microtubule accumulation at the predicted spindle poles during prophase and have reduced spindle bipolarity during prometaphase. Nonetheless, all abnormalities are rectified by anaphase and chromosome segregation appears normal. We conclude that ATK1 is required for normal microtubule accumulation at the spindle poles during prophase and possibly functions in spindle assembly during prometaphase. Because aberrant spindle morphology in these mutants is resolved by anaphase, we postulate that mitotic plant cells contain an error-correcting mechanism. Moreover, ATK1 function seems to be dosage-dependent, because cells containing one wild-type allele take significantly longer to proceed to anaphase as compared with cells containing two wild-type alleles

Cytoskeletal Motors in Arabidopsis. Sixty-One Kinesins and Seventeen Myosins

“…The atk1-1 mutation also prevents microtubule accumulation at mitotic spindle poles at early stages of spindle assembly (Marcus et al, 2003). Nevertheless, such an effect on microtubule organization during prometaphase does not affect the ultimate outcome of mitosis (Marcus et al, 2003).…”

A Minus-End–directed Kinesin with Plus-End Tracking Protein Activity Is Involved in Spindle Morphogenesis

“…Despite this abundance of Kinesin-14 family members, little is known about their roles with respect to spindle organization and function. It has been hypothesized that because plants do not contain classical microtubule organizing centers, such as centrosomes, the activity of motor proteins may play a central role in microtubule organization .The Arabidopsis Kinesin-14 family member ATK1 is involved in mitotic and meiotic spindle function (Liu et al, 1996;Chen et al, 2002;Marcus et al, 2002Marcus et al, , 2003. Atk1-1 null mutants exhibit splayed meiotic spindle poles, which result in defective chromosome segregation and reduced male fertility.…”

Mitosis-specific kinesins in Arabidopsis

“…The Arabidopsis genome contains 21 C-terminal kinesins now classified as Kinesin-14, of which seven are transcriptional and up-regulated at mitosis [6,8]. The Ncd-related ATK1 and ATK5 have been shown to move in the minusend direction of MTs and to associate with spindle MTs where they are required for focusing the MTs at the poles, which is similar to Ncd activity ( Figure 2) [33][34][35][36]. Compared with other organisms, the Kinesin-14 group is relatively large.…”

“…Spindle poles are broad in plant cells and poles are focused at the centrosomes in animal cells. Antagonistic forces of Kinesin-14, such as Ncd [81] and Arabidopsis ATK1 [35] and Kinesin-5, such as DmKLP61F [81], HsEg5 or AtTKRP125 establish spindle bipolarity, whereas Kinesin-14 members Ncd, ATK1 and ATK5 focus on the minus-ends of the spindle MTs [32][33][34][35][36]. Furthermore, members of the depolymerizing Kinesin-13 family (DmKLP10A and HsKIF2A) function in mitotic spindle assembly and maintenance [62].…”

“…How spindle bipolarity is accomplished is not known. In the absence of ATK1, the spindle poles are wider than normal but still bipolar, indicating that minus-end-directed kinesins are needed merely to focus the MTs at the poles [34,36]. Plant Kinesin-5 members, such as TKRP125, not only associate with the spindle but also with the phragmoplast [30].…”