Candida albicans cyclin Clb4 carries S-phase cyclin activity

Cell cycle regulation is a well-studied process in model organisms like Saccharomyces cerevisiae. These organisms use cyclin-dependent kinases (CDKs) to control progression through the cell cycle. Cyclins activate CDKs and may also influence substrate specificity. S. cerevisiae has nine cyclins, including three G1 cyclins and six B-type cyclins. B-type cyclins, such as Clb5 and Clb6, are active during the S phase. However, Candida albicans, a pathogenic yeast, lacks a direct homolog of Clb5/Clb6. This absence raises questions about how C. albicans regulates its cell cycle. Prior research has shown that cyclins from other species can sometimes substitute for missing ones in S. cerevisiae. That uncertainty drove the need to test whether C. albicans cyclins could functionally replace S. cerevisiae S-phase cyclins.

This study aimed to determine whether C. albicans cyclins could replace S. cerevisiae S-phase-specific cyclins. The researchers focused on CaClb2 and CaClb4, the two B-type cyclins in C. albicans. They also examined three G1 cyclins from C. albicans. The study tested whether these cyclins could activate CDKs and regulate the cell cycle in S. cerevisiae. The goal was to identify which C. albicans cyclin could substitute for Clb5/Clb6. The researchers also sought to understand the functional specificity of CaClb4. This work addresses a gap in understanding how cyclins from different species may have conserved or diverged roles in cell cycle control.

The researchers used in vitro assays to analyze the activity of CaClb2 and CaClb4. They also tested three G1 cyclins from C. albicans. The team introduced CaClb2 or CaClb4 into S. cerevisiae cells in place of Clb5. They monitored the resulting phenotypes, including cell cycle progression and DNA content. In vivo experiments involved observing the effects of cyclin expression on S. cerevisiae. The team also created a hybrid cyclin from CaClb2 and CaClb4 to test domain-specific functions. They assessed whether the cyclin box domain of CaClb4 was responsible for functional specificity. These approaches combined biochemical and genetic techniques to evaluate cyclin activity and specificity.

Replacement of Clb5 with CaClb4 in S. cerevisiae caused rapid diploidization. This suggests that CaClb4 can functionally replace Clb5/Clb6. In contrast, CaClb2 did not show the same S-phase activity. Both in vivo and in vitro analyses confirmed CaClb4's S-phase cyclin activity. The cyclin box domain of CaClb4 was found to carry functional specificity. A hybrid cyclin with the CaClb4 box domain retained activity, while others did not. These findings suggest that CaClb4 is the functional homolog of Clb5/Clb6. Despite higher sequence similarity between CaClb2 and Clb5/Clb6, CaClb4 was more effective in S-phase regulation.

The study shows that CaClb4 functions as an S-phase cyclin in S. cerevisiae. The authors propose that CaClb4 is the functional homolog of Clb5/Clb6. The cyclin box domain of CaClb4 appears to confer functional specificity. These findings suggest that CaClb4 may have evolved to take on roles similar to Clb5/Clb6 in C. albicans. The results also indicate that sequence similarity does not always predict functional equivalence. The hybrid cyclin experiments support the idea that the cyclin box domain is critical for activity. The authors suggest that these results provide insights into cyclin evolution and specificity. They emphasize the importance of functional assays in understanding cyclin roles across species.