Cell Biology: Deciphering the ABCs of SLiMs in G1-CDK Signaling

Abstract

A new study uses an elegant in vivo assay to comprehensively characterize the LP docking motif, which determines G1-CDK substrate specificity in fungi. The authors show that LP-cyclin docking strength determines the timing of Sic1 degradation, a key cell cycle event.

Figure 1.. SLiMs determine Cyclin–CDK substrate specificity and can be analyzed using an in vivo docking assay.

(A) Cyclin–CDK complexes that regulate cell cycle phases in S. cerevisiae (inner text) and vertebrates (outer text) are shown. Docking to late G1 cyclins (Cln1/2, red) is analyzed in [11]. (B) Mammalian CyclinA–CDK2 structure (PDB:1H26) illustrates cyclin substrate-binding surfaces. Salmon: Hydrophobic patch (hp) binds motifs in S (RxL), G2 (PxF) and M (LxF) phases. Red: Area on fungal G1 cyclins that binds LP motifs. Blue: CDK active site. (C) G1-CDK inhibits yeast mating to drive proliferation. Yeast pheromone binds to a G-protein-coupled receptor and activates a MAPK cascade (blue), to induce cell cycle arrest and mating. G1-CDK (yellow) phosphorylates the Ste5 scaffold (dark gray) and Ste20 kinase (light gray) to inhibit mating and drive proliferation (red lines); both contain LP motifs (yellow). Phosphorylation by G1-CDK (black circles), and MAPK pathway (blue circles) is shown. (D) in vivo assay for LP docking from [11]. G1-CDK (yellow) recognizes the Ste20–Ste5 chimeric protein (Ste20^ste5PM) via an LP motif (yellow) to inhibit mating and drive growth. Summary of workflow in [11]: LP motifs were systematically mutagenized and subjected to competitive growth, yielding a fitness score for each variant that was compiled in a position-specific scoring matrix, which statistically models sequence determinants at each motif position.