A Structural and Functional Analysis of the Dynein Light Intermediate Chain

Abstract

The concentrated cytoplasm of the cell must obtain spatial and temporal organization, and molecular motors are one means such organization can be achieved. Dynein is a multi-subunit microtubule motor complex that is implicated in a large range of functions including cargo transport and cell division. Many questions about dynein remain unanswered, including the mechanisms of cargo selection and attachment. The N-terminal tail subunits of dynein are crucial for cargo transport, yet it is not known how they select and attach cargo. In particular, the light intermediate chain (LIC) is the least studied tail subunit of dynein. Knockdown of the LIC leads to a number of mitotic and cargo-transport defects, and different LIC isoforms have demonstrated specificity in cargo selection. However, it is not understood how the LIC carries out these functions, and structural information about the LIC is lacking. We have solved the crystal structure of a conserved domain of cytoplasmic dynein LIC from a thermophilic fungus. Interestingly, this domain has a similar fold to GTPases. Despite having a Ras-like fold, the fungal LIC was crystallized without nucleotide bound and does not appear to bind nucleotide in solution. Based on this structure, we examined whether the human LIC would bind nucleotide since its sequence includes canonical G protein motifs unlike the fungal LIC. Interestingly the human LIC does in fact co-purify with a guanine nucleotide. Work is currently focused on conducting an in vivo mutational analysis of the LIC guided by the structure. Not only does our data render a more complete structural picture of dynein, but it also suggests a potentially interesting evolved biological function.