Biosynthesis and Characterization of the Brain-Specific Membrane Protein DPPX, a Dipeptidyl Peptidase IV--Related Protein

Abstract

Dipeptidyl peptidase IV-related protein (DPPX) was found to be preferentially expressed in the brain tissue. We isolated two rat cDNA clones encoding DPPX-S and DPPX-L from a brain cDNA library, of which DPPX-L had a longer sequence at the NH2 terminus. The biosynthesis of DPPXs was examined in both in vitro and in vivo systems. In the cell-free translation system, DPPX-S and DPPX-L were synthesized as 93-kDa and 97-kDa forms, respectively, which are in good agreement with the molecular masses estimated from their primary structure. In COS-1 cells transfected with the cDNAs, DPPX-S and DPPX-L were initially synthesized as 113-kDa and 117-kDa forms, respectively, with high-mannose type oligosaccharides, which were then converted to 115-kDa and 120-kDa forms, mostly with the complex-type sugar chains. Immunofluorescence-microscopic observations confirmed that both DPPXs were expressed on the cell surface. DPPXs were found to have no enzyme activity of DPPIV, even when they were mutated to have the consensus active-site sequence Gly-X-Ser-X-Gly for serine proteases. Immunoblot analysis of samples prepared from various rat tissues demonstrated that DPPX-S, but not DPPX-L, was detectable only in the brain tissue. These results indicate that, of the two isoforms, DPPX-S is preferentially expressed in the brain tissue as the surface glycoprotein without protease activity, although its function remains unknown at present.

DPP10 Modulates Kv4-mediated A-type Potassium Channels

“…5). Both DPPX and DPP10 have mutated catalytic sites, probably compromising the serine peptidase activity fundamental to the larger S9B prolyl oligopeptidase family (1,14). The sequence divergence within the hydrolase domain may reflect an evolutionary drift of DPPIVlike proteins that no longer function as enzymes.…”

“…Immunoblot Analysis-Immunoblots prepared as described previously (9) were incubated at 4°C for 14 h with anti-Kv4.2 polyclonal antibody (1:1000 dilution), anti-DPPX polyclonal antibody (1:1000 dilution) (14), or anti-DPP10 polyclonal antibody COO-12 (1:250 dilution; a generous gift of Dr. William Cookson). Bound antibodies were detected by chemiluminescence using an ECL detection kit (Pierce).…”

Structure of a Human A-type Potassium Channel Interacting Protein DPPX, a Member of the Dipeptidyl Aminopeptidase Family

“…[1][2][3] This family includes proteins such as DPP-IV 4 (CD26), fibroblast activation protein (FAP), 5 prolyl oligopeptidase (POP), 6 DPP8, 7 DPP9, and DPP10 (DPPY) 8 (K. Takimoto, personal communication). Prolyl oligopeptidases remove dipeptides from regulatory proteins and peptides.…”

Augmentation of Kv4.2-encoded Currents by Accessory Dipeptidyl Peptidase 6 and 10 Subunits Reflects Selective Cell Surface Kv4.2 Protein Stabilization

“…Co-expression with KChIPs increases the densities and alters the time-and voltage-dependent properties of Kv4 ␣ subunit-encoded currents (17, 24 -26). The accessory DPP6 and 10 subunits, in contrast, are transmembrane proteins (27) with high homology to the serine peptidase DPP4 (also referred to as CD26), but they lack proteolytic activity (28). Similar to the KChIPs, co-expression with DPP6 or DPP10 alters the properties and densities of Kv4-encoded currents (15, 29 -31).…”