Proteomic Analysis of Lysosomal Membrane Proteins in Bovine Mammary Epithelial Cells Illuminates Potential Novel Lysosome Functions in Lactation

Lactation of bovine
mammary epithelial cells (BMEC) is a complex
biological process that involves in various organelles. Studies have
shown that lysosome and lysosomal membrane proteins (LMP) plays an
important role in lactation of BMEC. But the LMP of BMEC remains poorly
understood. To obtain a global view of the LMP of BMEC and the affect
of lysosome on lactation, the LMP of BMEC was identified using sequential
windowed acquisition of all theoretical mass spectra (LC-SWATH/MS).
1214 LMP were identified and 559 were reported to be localized on
lysosomal membrane for the first time in BMEC. Gene ontology annotation
of these identified proteins showed that both previously reported
casein synthesis-related LMP, such as LAMTOR1, 2, 3, and rRagC, and
newly identified casein and milk fat synthesis-related LMP, such as
EIF4E and ACAA1, were found. KEGG pathway analysis of these identified
proteins showed that some pathways involved in lactation, such as
PI3K-Akt, mTOR, insulin, PPAR, and JAK-STAT pathway, were found. The
lysosomal location of five proteins (PRKCA, EIF4E, ACAA1, HRAS, and
THBS1) was analyzed by laser confocal microscopy, and all five were
associated with the lysosomal membrane. These findings help to elucidate
lysosome functions in the regulation of lactation. The results implicate
lysosomes as important organelles in regulation of lactation of BMEC
that have been previously undervalued.