The spindle assembly checkpoint monitors microtubule attachment to kinetochores and tension across sister kinetochores to ensure accurate division of chromosomes between daughter cells. Cytoplasmic dynein functions in the checkpoint, apparently by moving critical checkpoint components off kinetochores. The dynein subunit required for this function is unknown. Here we show that human cells depleted of dynein light intermediate chain 1 (LIC1) delay in metaphase with increased interkinetochore distances; dynein remains intact, localised and functional. The checkpoint proteins Mad1/2 and Zw10 localise to kinetochores under full tension, whereas BubR1 is diminished at kinetochores. Metaphase delay and increased interkinetochore distances are suppressed by depletion of Mad1, Mad2 or BubR1 or by re-expression of wtLIC1 or a Cdk1 site phosphomimetic LIC1 mutant, but not Cdk1-phosphorylation-deficient LIC1. When the checkpoint is activated by microtubule depolymerisation, Mad1/2 and BubR1 localise to kinetochores. We conclude that a Cdk1 phosphorylated form of LIC1 is required to remove Mad1/2 and Zw10 but not BubR1 from kinetochores during spindle assembly checkpoint silencing.

Cells as dynamic, interactive, and self contained units of life have a need for molecular motors that can create physical forces to move cargoes within the cell. Cytoplasmic dynein 1 is one such molecular motor that has many functions in the cell. The number and variety of functions that involve cytoplasmic dynein 1 suggest that there are a number of different binding sites on dynein for different proteins. Cytoplasmic dynein 1 is a multiprotein complex made up of six different subunit families. The many different combinations of subunits that could be used to make up a cytoplasmic dynein 1 holocomplex provides the variety of different binding sites for cargoes that can be individually regulated. The following chapters flush out how light intermediate chain 1 (LIC1), a subunit of cytoplasmic dynein 1, is involved with multiple dynein functions involving the binding of different cargoes to the cytoplasmic dynein 1 holocomplex, and how the binding of these cargoes can be regulated. First, LIC1 is found to be involved in the spindle assembly checkpoint. LIC1 appears to facilitate the removal of Mad1-Mad2, a complex important in producing a wait anaphase signal, from kinetochores. Second, the involvement of LIC1 in the spindle assembly checkpoint requires the phosphorylation of LIC1 at a putative Cdk1 phosphorylation site. This site is located in a domain of LIC1 that binds various proteins suggesting that this phosphorylation could also regulate these interactions. Third, LIC1 is involved in the centrosomal assembly of pericentrin, an important centrosomal protein. From the data presented herein, LIC1 is shaping up as a multifunctional cargo binder for cytoplasmic dynein 1 that requires regulation of its various cargoes.