Structural and Functional Investigation of SAM68 and T-STAR, with an Emphasis on the SAM68 and T-STAR Complex

The progression of some diseases can be linked to the dysregulation of protein activity. SAM68 and T-STAR are members of the STAR family of proteins which have been shown to link signal transduction with RNA metabolism: including RNA transport, translational regulation, and pre-mRNA alternative splicing. Overexpression of SAM68 and T-STAR has been linked to some cancers and, furthermore, the dysregulation of these proteins can alter pre-mRNA alternative splicing outcomes. SAM68 and T-STAR can form homodimers and they have been shown to interact with each other to form a SAM68/T-STAR complex. However, the structural basis and functional significance of the SAM68/T-STAR complex is unknown. The structure of the complex was studied by 2D NMR using a fusion protein of the T-STAR and SAM68 QUA1-KH domains. These results show that the SAM68/T-STAR complex can form via the QUA1-KH domains and that the complex contains a mixture of SAM68-SAM68, T-STAR-T-STAR, and SAM68-T-STAR interactions which is possible by the formation of higher-order multimers. Single-molecule studies also showed the presence of higher-order multimers for the full-length SAM68 and T-STAR proteins and multimers were further driven by the addition of Bcl-x and G8-5 RNA. These results contrast with previous studies showing that the free and RNA-bound SAM68 and T-STAR proteins form homodimers. Therefore, the results here illustrate a new mechanism for the formation of SAM68- and T-STAR-containing complexes and could provide a new basis for understanding the function of these proteins.