Doctor of Philosophy (PhD)


Biological Sciences

Document Type



SIAMESE (SIM) and SIAMESE-RELATED-PROTEIN1 (SMR1), the founding members of the SIM/SMRs gene family, suppress mitosis and onset of endoreplication in the Arabidopsis’s trichome and sepal development, respectively, and hence have been suggested to be CDK inhibitors. In this study, I have investigated the exact role of SIM and SMRs and their evolutionarily conserved function throughout land plant evolution. Using split luciferase complementation (SLC), I have shown that both SIM and a distantly related a bryophyte “Physcomitrella patens” SMR (pSMR1) interacts with multiple types of Cyclin Dependent Kinases (CDKs). I have multiple lines of evidence that establish SIM and SMRs as CDKs inhibitors and demonstrating that their evolutionary function is conserved. Almost all SIAMESE-RELATED PROTEINS (SMRs) of Arabidopsis as well as a SMR from the bryophyte Physcomitrella patens complement the sim mutant phenotype strongly. Genetic studies of sim mutants in combination with cyclind and cdkb1 mutants also support the conclusion that SIM inhibits the activity of both CDKA;1 and CDKB1;1-containing complexes. In an in vitro kinase assay, SIM inhibits CDK kinase activity; moreover, the Physcomitrella SMR also inhibits the same set of CYC/CDK complexes as SIM. These results indicate that SIM and other SMRs inhibit multiple CDK complexes and share a molecular mechanism that is conserved among all land plants. Finally, we have investigated the functional role of conserved protein sequence motifs in SIM. Two motifs, termed Motif-1 and Motif-2, play important roles in SIM function. Surprisingly, a motif previously thought to be a putative cyclin-binding motif is not essential for function of SIM. We have also identified a putative CDK phosphorylation site in Motif-1, and two nuclear localization sequences that are essential for SIM function. The work described here gives new insights into the biochemical role of SIM in regulating the cell cycle. The conserved function of widely divergent SMRs indicates that this protein family plays important roles in all land plants. These studies will provide a foundation for future work on the biochemical functions of SIM in the cell cycle, as well as for understanding the roles of individual SMRs in plant growth and development.



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Committee Chair

Larkin, John C