Goodpasture-antigen binding protein (GPBP) is a nonconventional Ser/Thr kinase for basement membrane type IV collagen.Various studies have questioned these findings and proposed that GPBP serves as transporter of ceramide between the endoplasmic reticulum and the Golgi apparatus. Here we show that cells expressed at least two GPBP isoforms resulting from canonical (77-kDa) and noncanonical (91-kDa) mRNA translation initiation. The 77-kDa polypeptide interacted with type IV collagen and localized as a soluble form in the extracellular compartment. The 91-kDa polypeptide and its derived 120-kDa polypeptide associated with cellular membranes and regulated the extracellular levels of the 77-kDa polypeptide. A short motif containing two phenylalanines in an acidic tract and the 26-residue Ser-rich region were required for efficient 77-kDa polypeptide secretion. Removal of the 26-residue Ser-rich region by alternative exon splicing rendered the protein cytosolic and sensitive to the reduction of sphingomyelin cellular levels. These and previous data implicate GPBPs in a multicompartmental program for protein secretion (i.e. type IV collagen) that includes: 1) phosphorylation and regulation of protein molecular/supramolecular organization and 2) interorganelle ceramide trafficking and regulation of protein cargo transport to the plasma membrane. Goodpasture antigen-binding protein (GPBP)2 phosphorylates the noncollagenous-1 (NC1) domain of the ␣3 chain of type IV collagen (␣3(IV)NC1) (1). This domain is a pivotal structure in the molecular and supramolecular organization of the glomerular basement membrane collagen and also the target of autoantibodies mediating glomerulonephritis in Goodpasture disease (2). Increased GPBP expression has been associated with autoimmune pathogenesis including Goodpasture disease (3) and with the induction of glomerular basement membrane collagen disorganization and deposit of IgA antibodies (4). These observations suggest that GPBP regulates glomerular basement membrane collagen organization and induces type IV collagen-based antibody-mediated glomerulonephritis when its expression is abnormally elevated (3, 4). The GPBP gene (COL4A3BP) also encodes for GPBP⌬26, a more abundant, less active, alternatively spliced GPBP variant lacking a 26-residue Ser-rich region that is apparently not regulated under these pathological conditions (3).GPBP contains multiple structural elements including N-terminal pleckstrin homology domain, Ser-Xaa-Yaa region, bipartite nuclear localization signal, coiled-coil domain, two phenylalanines in an acidic tract (FFAT) motif, and C-terminal steroidogenic acute regulatory related lipid transfer (START) domain. Additional structural features include motifs for selfinteraction and phosphorylation (1, 3, 5, 6). The pleckstrin homology domains comprise a variety of poorly conserved structures present only in eukaryotes that have been proposed to mediate protein targeting to cellular membranes through interaction with phosphoinositides (7). A variety of proteins inc...

Goodpasture antigen-binding protein (GPBP) is an exportable1 Ser/Thr kinase that induces collagen IV expansion and has been associated with chemoresistance following epithelial-to-mesenchymal transition (EMT). Here we demonstrate that cancer EMT phenotypes secrete GPBP (mesenchymal GPBP) which displays a predominant multimeric oligomerization and directs the formation of previously unrecognized mesh collagen IV networks (mesenchymal collagen IV). Yeast twohybrid (YTH) system was used to identify a 260 SHCIE 264 motif critical for multimeric GPBP assembly which then facilitated design of a series of potential peptidomimetics. The compound 3-[4''-methoxy-3,2'-dimethyl-(1,1';4',1'')terphenyl-2''-yl]propionic acid, or T12, specifically targets mesenchymal GPBP and disturbs its multimerization without affecting kinase catalytic site. Importantly, T12 reduces growth and metastases of tumors populated by EMT phenotypes. Moreover, low-dose doxorubicin sensitizes epithelial cancer precursor cells to T12, thereby further reducing tumor load. Given that T12 targets the pathogenic mesenchymal GPBP, it does not bind significantly to normal tissues and therapeutic dosing was not associated with toxicity. T12 is a first-in-class drug candidate to treat cancer by selectively targeting the collagen IV of the tumor cell microenvironment. www.impactjournals.com/oncotarget/

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