Cloning and characterization of a novel human phosphatidylinositol transfer protein, rdgB beta

The various PITP, retinal degeneration B (rdgB), and amino-terminal domain interacting receptor (Nir) phosphatidylinositol transfer proteins can be di vided into two structural families. The small, soluble PITP isoforms contai n only a phosphatidylinositol transfer domain and have been implicated in p hosphoinositide signaling and vesicle trafficking. In contrast, the rdgB pr oteins, which include Nir2 and Nir3, contain an amino-terminal PITP-like do main, an acidic, Ca2+-binding domain, six putative transmembrane domains, a nd a conserved carboxyl-terminal domain. However, the biological function o f rdgB proteins is unclear, Here, we report the isolation of a cDNA encodin g a novel rdgB protein, mammalian rdgB beta (MrdgB beta). The 38-kDa MrdgB beta protein contains an amino-terminal PITP-like domain and a short carbox yl-terminal domain. In contrast to other rdgB-like proteins, MrdgB beta con tains no transmembrane motifs or the conserved carboxyl-terminal domain. Us ing Northern and reverse transcription-polymerase chain reaction analysis, we demonstrate that MrdgB beta mRNA is ubiquitously expressed. Immunofluore scence analysis of ectopic MrdgB beta showed cytoplasmic staining, and the ability of recombinant MrdgB beta to transfer phosphatidylinositol in vitro was similar to other PITP-like domains. Although early reports found funct ional degeneracy in, vitro, the identification of a fifth mammalian PITP-li ke protein with a unique domain organization and widespread expression supp orts more recent results that suggest that different PITP-like domains have distinct functions in vivo.