Abstract
The yeast Spf1p protein is a primary transporter that belongs to group 5 of the large family of P-ATPases. Loss of Spf1p function produces ER stress with alterations of metal ion and sterol homeostasis and protein folding, glycosylation and membrane insertion. The amino acid sequence of Spf1p shows the characteristic P-ATPase domains A, N, and P and the transmembrane segments M1-M10. In addition, Spf1p exhibits unique structures at its N-terminus (N-T region), including two putative additional transmembrane domains, and a large insertion connecting the P domain with transmembrane segment M5 (D region). Here we used limited proteolysis to examine the structure of Spf1p. A short exposure of Spf1p to trypsin or proteinase K resulted in the cleavage at the N and C terminal regions of the protein and abrogated the formation of the catalytic phosphoenzyme and the ATPase activity. In contrast, limited proteolysis of Spf1p with chymotrypsin generated a large N-terminal fragment containing most of the M4-M5 cytosolic loop, and a minor fragment containing the C-terminal region. If lipids were present during chymotryptic proteolysis, phosphoenzyme formation and ATPase activity were preserved. ATP slowed Spf1p proteolysis without detectable changes of the generated fragments. The analysis of the proteolytic peptides by mass spectrometry and Edman degradation indicated that the preferential chymotryptic site was localized near the cytosolic end of M5. The susceptibility to proteolysis suggests an unexpected exposure of this region of Spf1p that may be an intrinsic feature of P5A-ATPases.
Highlights
P5-ATPases are primary active transporters present in all eukaryotic cells [1,2,3]
The fusion of GFP to a protein is useful as a tool for limited proteolysis analysis because GFP is relatively resistant to proteases and it may generate identifiable fluorescent peptides derived from the target protein
These results show that despite the specificity of the particular protease, the initial proteolytic digestion of GFP-sensitivity to Pichia farinosa killer toxin protein (Spf1p) involved the cleavage of the protein ends generating large fragments, that judging by its size, would include the catalytic core
Summary
P5-ATPases are primary active transporters present in all eukaryotic cells [1,2,3]. Dysfunction of P5-ATPases affects the function of ER, lysosomes, peroxisomes, and mitochondria, in parallel with changes in the lipid and metal ion homeostasis [4,5,6,7,8,9,10,11,12,13,14,15]. The P5A-ATPases have been proposed to transport metal ions like Ca2+, Mg2+ and Mn2+ and sterols [6, 9, 11, 14]. Recent data shows that P5A-ATPases function as transmembrane helix dislocases suggesting that the transported substrate is of peptidyl nature [22, 23]. The Spf1p, the best characterized P5A-ATPase, is a protein of Mr~135 kDa localized in the yeast ER. Purified preparations of recombinant Spf1p have been shown to be capable of ATP hydrolysis and to form maximal levels of the characteristic P-ATPase catalytic phosphoenzyme in the absence of metal ions, with the exception of Mg2+ [6, 14, 24, 25]
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