The study of peroxisome biogenesis seeks to understand the underlying processes involved in the initial formation, maturation, and proliferation of peroxisomes. The two peroxins, Pex3 and Pex19, have essential functions in peroxisome biogenesis; cells lacking Pex3 or Pex19 have no detectable mature peroxisomes. Several studies have highlighted the importance of the endoplasmic reticulum (ER) in the de novo synthesis of peroxisomes, contrary to previous growth and division models. More recently, the novel Pex protein, Pex25, has also been implicated as a requirement for the de novo pathway. Nevertheless, the mechanism by which these proteins promote the de novo process is currently unknown. In this study, we have characterized several Pex19 N-and C-terminal deletions to determine the structural domains of the peroxin required for proper peroxisome membrane protein (PMP) and peroxisome assembly. Deletions in the N-terminal Pex3 binding site weakened Pex19's interactions with Pex3 but preserved interactions with Pex12, a mPTS-containing PMP known to bind with the C-terminal end of Pex19. For these constructs only, Pex25 was essential for de novo peroxisome biogenesis. In contrast, deletions in the C- terminal mPTS-binding domain of Pex19 weakened the interaction between Pex19 and Pex12, while leaving Pex19- Pex3 interactions intact. Surprisingly, import competent peroxisomes were formed in both sets of deletions although a delay of 12-18 hours in peroxisome biogenesis was observed. This delay was decreased upon the overexpression of Pex25 for both N-and C-terminal deletion mutants. Co- immunoprecipitation studies revealed that Pex25 promotes peroxisome biogenesis through strengthening Pex19 and PMP interactions, which are required for the exit of pre- peroxisomal vesicular carriers. In conclusion, the physical segregation of Pex19's Pex3 and PMP binding domains has provided novel insights into the role of Pex19 in peroxisome biogenesis, via the interaction of a central domain in Pex19 with Pex25

The peroxins Pex19 and Pex3 play an indispensable role in peroxisomal membrane protein (PMP) biogenesis, peroxisome division, and inheritance. Pex19 plays multiple roles in these processes, but how these functions relate to the structural organization of the Pex19 domains is unresolved. To this end, using deletion mutants, we mapped the Pex19 regions required for peroxisome biogenesis in the yeast Pichia pastoris Surprisingly, import-competent peroxisomes still formed when Pex19 domains previously believed to be required for biogenesis were deleted, although the peroxisome size was larger than that in wild-type cells. Moreover, these mutants exhibited a delay of 14-24 h in peroxisome biogenesis. The shortest functional N-terminal (NTCs) and C-terminal constructs (CTCs) were Pex19 (aa 1-150) and Pex19 (aa 89-300), respectively. Deletions of the N-terminal Pex3-binding site disrupted the direct interactions of Pex19 with Pex3, but preserved interactions with a membrane peroxisomal targeting signal (mPTS)-containing PMP, Pex10. In contrast, deletion of the C-terminal mPTS-binding domain of Pex19 disrupted its interaction with Pex10 while leaving the Pex19-Pex3 interactions intact. However, Pex11 and Pex25 retained their interactions with both N- and C-terminal deletion mutants. NTC-CTC co-expression improved growth and reversed the larger-than-normal peroxisome size observed with the single deletions. Pex25 was critical for peroxisome formation with the CTC variants, and its overexpression enhanced their interactions with Pex3 and aided the growth of both NTC and CTC Pex19 variants. In conclusion, physical segregation of the Pex3- and PMP-binding domains of Pex19 has provided novel insights into the modular architecture of Pex19. We define the minimum region of Pex19 required for peroxisome biogenesis and a unique role for Pex25 in this process.

Polygenic scores (PGSs) summarize the combined effect of common risk variants and are associated with breast cancer risk in patients without identifiable monogenic risk factors. One of the most well-validated PGSs in breast cancer to date is PGS₃₁₃, which was developed from a Northern European biobank but has shown attenuated performance in non-European ancestries. We further investigate the generalizability of the PGS₃₁₃ for American women of European (EA), African (AFR), Asian (EAA), and Latinx (HL) ancestry within one institution with a singular electronic health record (EHR) system, genotyping platform, and quality control process. We found that the PGS₃₁₃ achieved overlapping areas under the receiver operator characteristic (ROC) curve (AUCs) in females of HL (AUC = 0.68, 95% confidence interval [CI] = 0.65-0.71) and EA ancestry (AUC = 0.70, 95% CI = 0.69-0.71) but lower AUCs for the AFR and EAA populations (AFR: AUC = 0.61, 95% CI = 0.56-0.65; EAA: AUC = 0.64, 95% CI = 0.60-0.680). While PGS₃₁₃ is associated with hormone-receptor-positive (HR+) disease in EA Americans (odds ratio [OR] = 1.42, 95% CI = 1.16-1.64), this association is lost in African, Latinx, and Asian Americans. In summary, we found that PGS₃₁₃ was significantly associated with breast cancer but with attenuated accuracy in women of AFR and EAA descent within a singular health system in Los Angeles. Our work further highlights the need for additional validation in diverse cohorts prior to the clinical implementation of PGSs.