Differential expression of a C-terminal splice variant of phosphatidylinositol transfer protein β lacking the constitutive-phosphorylated Ser262 that localizes to the Golgi compartment

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Abstract
Mammalian PITPβ (phosphatidylinositol transfer protein β) is a 272-amino-acid polypeptide capable of transferring PtdIns, PtdCho and SM (sphingomyelin) between membrane bilayers. It has been reported that Ser262 present in the C-terminus of PITPβ is constitutively phosphorylated and determines Golgi localization. We provide evidence for the expression of an sp (splice) variant of PITPβ (PITPβ-sp2) where the C-terminal 15 amino acids of PITPβ-sp1 are replaced by an alternative C-terminus of 16 amino acids. PITPβ-sp1 is the product of the first 11 exons, whereas PITPβ-sp2 is a product of the first 10 exons followed by the twelfth exon – exon 11 being ‘skipped’. Both splice variants are capable of PtdIns and PtdCho transfer, with PITPβ-sp2 being unable to transport SM. PITPβ is ubiquitously expressed, with the highest amounts of PITPβ found in HL60 cells and in rat liver; HL60 cells express only PITPβ-sp1, whereas rat liver expresses both sp variants in similar amounts. In both cell types, PITPβ-sp1 is constitutively phosphorylated and both the PtdIns and PtdCho forms of PITPβ-sp1 are present. In contrast, PITPβ-sp2 lacks the constitutively phosphorylated Ser262 (replaced with glutamine). Nonetheless, both PITPβ variants localize to the Golgi and, moreover, dephosphorylation of Ser262 of PITPβ-sp1 does not affect its Golgi localization. The presence of PITPβ sp variants adds an extra level of proteome complexity and, in rat liver, the single gene for PITPβ gives rise to seven distinct protein species that can be resolved on the basis of their charge differences.