Distinct proteomic profile of ovarian follicular fluid in ewes from small versus large developing follicles

•Fifty-five major proteins were identified in the 2 groups.•The gene ontology identified as the main molecular function - catalytic activity and binding, as the main biological processes - the cellular and metabolic processes, and as the main cellular component - the cell part.•Only 2 proteins were found in higher abundance in follicles < 4 mm.•In the group of follicles ≥ 4 to 6 mm, 10 proteins were in higher abundance. This study aimed to describe the proteins of the ovarian follicular fluid from ewes. The follicular fluid was taken from 294 ovaries collected from slaughterhouses and split into two groups based on follicle size: Group 1 (< 4 mm); and Group 2 (≥ 4 to 6 mm). The samples for ESI-Q-TOF MS/MS mass spectrometry were prepared through Tryptic in-gel digestion. For comparisons between groups, the data were analyzed using uni and multivariate statistical analysis, represented by principal component analysis (PCA), the measure of a variable's importance (VIP score) in the partial least squares-discriminant analysis (PLS-DA), the fold change, and t-test, using the exponentially modified protein abundance index (emPAI). Moreover, gene ontology was performed to identify molecular function, biological process, and cellular component. Fifty-five major proteins were identified in the 2 groups. The separation of the formed clusters was clearly observed in the PCA. Twelve proteins had a VIP score with α ≥ 1.5. Only 2 proteins were found in higher abundance in Group 1, the serum amyloid A-4 protein isoform X1 and trinucleotide repeat-containing gene 6c protein isoform X2. However, in Group 2, 10 proteins were in higher abundance: complement component C6, serotransferrin isoform X2, fibrinopeptide A, tetranectin, inter-alpha-trypsin inhibitor heavy chain H4 isoform X1, serpin B9 isoform X2, alpha-enolase isoform X6, serum paraoxonase/arylesterase 1, complement factor B, and complement C3 isoform X2, partial. We concluded, therefore, that a distinct profile of protein abundance is observed in small vs large growing follicles, including proteins especially related to ovulation, follicular metabolism, and oocyte development and quality. [Display omitted]