Francisco Revert-Ros

Goodpasture Antigen-binding Protein Is a Soluble Exportable Protein That Interacts with Type IV Collagen IDENTIFICATION OF NOVEL MEMBRANE-BOUND ISOFORMS

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.

A human-specific TNF-responsive promoter for Goodpasture antigen-binding protein

The Goodpasture antigen‐binding protein, GPBP, is a serine/threonine kinase whose relative expression increases in autoimmune processes. Tumor necrosis factor (TNF) is a pro‐inflammatory cytokine implicated in autoimmune pathogenesis. Here we show that COL4A3BP, the gene encoding GPBP, maps head‐to‐head with POLK, the gene encoding for DNA polymerase kappa (pol κ), and shares with it a 140‐bp promoter containing a Sp1 site, a TATA‐like element, and a nuclear factor kappa B (NFκB)‐like site. These three elements cooperate in the assembly of a bidirectional transcription complex containing abundant Sp1 and little NFκB that is more efficient in the POLK direction. Tumour necrosis factor cell induction is associated with Sp1 release, NFκB recruitment and assembly of a complex comparatively more efficient in the COL4A3BP direction. This is accomplished by competitive binding of Sp1 and NFκB to a DNA element encompassing a NFκB‐like site that is pivotal for the 140‐bp promoter to function. Consistently, a murine homologous DNA region, which contains the Sp1 site and the TATA‐like element but is devoid of the NFκB‐like site, does not show transcriptional activity in transient gene expression assays. Our findings identify a human‐specific TNF‐responsive transcriptional unit that locates GPBP in the signalling cascade of TNF and substantiates previous observations, which independently related TNF and GPBP with human autoimmunity.

Goodpasture Antigen-binding Protein (GPBP) Directs Myofibril Formation: IDENTIFICATION OF INTRACELLULAR DOWNSTREAM EFFECTOR 130-kDa GPBP-INTERACTING PROTEIN (GIP130)*

Background: GPBP-1 is a non-conventional kinase that regulates glomerular basement membrane collagen organization. Results: GPBP-1 targets GIP130, a new myosin-binding protein, and regulates myofibrillogenesis in cultured myoblasts. Conclusion: GPBP-1 regulates myofibril formation. Significance: GPBP-1 is a kinase for structural protein organization at both intracellular and extracellular compartments. Goodpasture antigen-binding protein-1 (GPBP-1) is an exportable non-conventional Ser/Thr kinase that regulates glomerular basement membrane collagen organization. Here we provide evidence that GPBP-1 accumulates in the cytoplasm of differentiating mouse myoblasts prior to myosin synthesis. Myoblasts deficient in GPBP-1 display defective myofibril formation, whereas myofibrils assemble with enhanced efficiency in those overexpressing GPBP-1. We also show that GPBP-1 targets the previously unidentified GIP130 (GPBP-interacting protein of 130 kDa), which binds to myosin and promotes its myofibrillar assembly. This report reveals that GPBP-1 directs myofibril formation, an observation that expands its reported role in supramolecular organization of structural proteins to the intracellular compartment.