BRN1

Brn1 is a non-SMC subunit of the condensin complex in the budding yeast Saccharomyces cerevisiae. The BRN1 gene encodes a protein that associates with the core SMC subunits Smc2 and Smc4, as well as other non-SMC subunits, to form the functional condensin complex responsible for chromosome condensation and structural organization during the cell cycle.

In function, Brn1 contributes to the condensation of chromosomes during mitosis and meiosis, enabling proper chromosome compaction, sister chromatid resolution, and faithful segregation to daughter cells. Loss or impairment of Brn1 function leads to defects in chromosome condensation, increased chromosome missegregation, and growth defects, highlighting its essential role in maintaining genome stability.

Structurally, Brn1 acts as an accessory subunit that helps recruit and stabilize the condensin complex on chromatin. It interacts with the core SMC proteins Smc2 and Smc4 and with other condensin subunits, contributing to complex assembly, chromatin association, and the regulation of condensin activity during the cell cycle.

Localization studies show Brn1 primarily in the nucleus, with chromatin-associated localization that peaks during mitosis. Its activity is coordinated with cell cycle progression to ensure timely condensation and subsequent chromosome segregation.

Evolutionarily, Brn1 is considered the fungal homolog of CAP-H, the non-SMC subunit of condensin I in higher eukaryotes, reflecting the conserved role of condensin complexes in chromosome condensation across diverse species. In yeast, Brn1 and its condensin partners constitute a single, essential complex critical for genome maintenance and cell viability.