Morgan Clive P, Allen-Baume Victoria, Radulovic Marko, Li Michelle, Skippen Alison, Cockcroft Shamshad
Lipid Signalling Group, Department of Physiology, University College London, 21 University Street, Rockefeller Building, London WC1E 6JJ, UK.
Biochem J. 2006 Sep 15;398(3):411-21. doi: 10.1042/BJ20060420.
Mammalian PITPbeta (phosphatidylinositol transfer protein beta) 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 PITPbeta is constitutively phosphorylated and determines Golgi localization. We provide evidence for the expression of an sp (splice) variant of PITPbeta (PITPbeta-sp2) where the C-terminal 15 amino acids of PITPbeta-sp1 are replaced by an alternative C-terminus of 16 amino acids. PITPbeta-sp1 is the product of the first 11 exons, whereas PITPbeta-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 PITPbeta-sp2 being unable to transport SM. PITPbeta is ubiquitously expressed, with the highest amounts of PITPbeta found in HL60 cells and in rat liver; HL60 cells express only PITPbeta-sp1, whereas rat liver expresses both sp variants in similar amounts. In both cell types, PITPbeta-sp1 is constitutively phosphorylated and both the PtdIns and PtdCho forms of PITPbeta-sp1 are present. In contrast, PITPbeta-sp2 lacks the constitutively phosphorylated Ser262 (replaced with glutamine). Nonetheless, both PITPbeta variants localize to the Golgi and, moreover, dephosphorylation of Ser262 of PITPbeta-sp1 does not affect its Golgi localization. The presence of PITPbeta sp variants adds an extra level of proteome complexity and, in rat liver, the single gene for PITPbeta gives rise to seven distinct protein species that can be resolved on the basis of their charge differences.
哺乳动物的PITPβ（磷脂酰肌醇转移蛋白β）是一种由272个氨基酸组成的多肽，能够在膜双层之间转移磷脂酰肌醇（PtdIns）、磷脂酰胆碱（PtdCho）和鞘磷脂（SM）。据报道，PITPβ C末端的丝氨酸262（Ser262）持续磷酸化并决定高尔基体定位。我们提供了PITPβ的一个剪接变体（PITPβ-sp2）表达的证据，其中PITPβ-sp1的C末端15个氨基酸被16个氨基酸的另一种C末端所取代。PITPβ-sp1是前11个外显子的产物，而PITPβ-sp2是前10个外显子与第12个外显子的产物——第11个外显子被“跳过”。两种剪接变体都能够进行PtdIns和PtdCho转移，而PITPβ-sp2不能转运SM。PITPβ在各处均有表达，在HL60细胞和大鼠肝脏中含量最高；HL60细胞仅表达PITPβ-sp1，而大鼠肝脏以相似的量表达两种剪接变体。在这两种细胞类型中，PITPβ-sp1持续磷酸化，且PITPβ-sp1的PtdIns和PtdCho形式均存在。相比之下，PITPβ-sp2缺乏持续磷酸化的Ser262（被谷氨酰胺取代）。尽管如此，两种PITPβ变体都定位于高尔基体，此外，PITPβ-sp1的Ser262去磷酸化并不影响其高尔基体定位。PITPβ剪接变体的存在增加了蛋白质组复杂性的一个额外层次，并且在大鼠肝脏中，PITPβ的单个基因产生了七种不同的蛋白质种类，可根据它们的电荷差异进行区分。
