title: BRI2 and BRI3 are functionally distinct phosphoproteins.
abstract: Three BRI protein family members have been identified. Among these are BRI3 and BRI2, the latter is associated with Familial Danish and Familial British dementias. 'In silico' sequence analysis identified putative PP1 binding sites in BRI2 and BRI3. This is singularly important, given that protein phosphorylation is a major mechanism regulating intracellular processes. Protein phosphatase 1 (PP1) interacting proteins (PIPs) are fundamental in determining substrate specificity and subcellular localization of this phosphatase. More than 200 PIPs have thus far been reported. Both BRI2 and BRI3 are type II transmembrane glycoproteins relevant in neuronal systems. Using Myc-BRI2 and Myc-BRI3, wild type and PP1 binding mutant constructs, it was possible to show, for the first time, that in fact BRI2 and BRI3 bind PP1. The complexes BRI2:PP1 and BRI3:PP1 were validated in vitro and in vivo. The subcellular distribution of BRI2 and BRI3 is similar; both localize to the perinuclear area and Golgi apparatus in non-neuronal cells. However, in SH-SY5Y cells, BRI2 and BRI3 could also be detected in elongated cellular projections ('processes') and in rat cortical neurons both are broadly distributed throughout the cell body, neuritis and the nucleus. Consistently, co-localization of BRI2 and BRI3 with PP1 was evident. The functional significance of these complexes is apparent given that both BRI proteins are substrates of PP1, thus simultaneously this is the first report of BRI2 and BRI3 as phosphoproteins. Moreover, we show that when BRI2 is phosphorylated a significant increase in neuronal outgrowth and differentiation is evident. Interestingly, the Alzheimer's amyloid precursor protein (APP), forms a trimeric complex composed of PP1 and Fe65, with PP1 having the capacity to dephosphorylate APP at Thr668 residue. The emerging consensus appears to be that PP1 containing complexes are crucial in regulating signaling events underlying neuropathological conditions.
authors.affiliation: Laboratório de Neurociências e Sinalização Celular, Centro de Biologia Celular, iBiMED, SACS, Universidade de Aveiro, Aveiro, Portugal.
authors.forename: Filipa
authors.initials: F
authors.lastname: Martins
authors.affiliation: Laboratório de Neurociências e Sinalização Celular, Centro de Biologia Celular, iBiMED, SACS, Universidade de Aveiro, Aveiro, Portugal. Electronic address: srebelo@ua.pt.
authors.forename: Sandra
authors.initials: S
authors.lastname: Rebelo
authors.affiliation: Laboratório de Neurociências e Sinalização Celular, Centro de Biologia Celular, iBiMED, SACS, Universidade de Aveiro, Aveiro, Portugal.
authors.forename: Mariana
authors.initials: M
authors.lastname: Santos
authors.affiliation: Laboratório de Neurociências e Sinalização Celular, Centro de Biologia Celular, iBiMED, SACS, Universidade de Aveiro, Aveiro, Portugal.
authors.forename: Cândida Zita
authors.initials: CZ
authors.lastname: Cotrim
authors.affiliation: Laboratório de Neurociências e Sinalização Celular, Centro de Biologia Celular, iBiMED, SACS, Universidade de Aveiro, Aveiro, Portugal.
authors.forename: Edgar F
authors.initials: EF
authors.lastname: da Cruz e Silva
authors.affiliation: Laboratório de Neurociências e Sinalização Celular, Centro de Biologia Celular, iBiMED, SACS, Universidade de Aveiro, Aveiro, Portugal.
authors.forename: Odete A B
authors.initials: OA
authors.lastname: da Cruz e Silva
mesh_terms: D048868:Adaptor Proteins, Signal Transducing; D000544:Alzheimer Disease / Q000378:metabolism; D016229:Amyloid beta-Peptides / Q000378:metabolism; D016564:Amyloid beta-Protein Precursor / Q000378:metabolism; D000818:Animals; D001921:Brain / Q000378:metabolism; D006801:Humans; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D009154:Mutation / Q000235:genetics; D009474:Neurons / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism; D017208:Rats, Wistar
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D016229:Amyloid beta-Peptides; D016564:Amyloid beta-Protein Precursor; C120083:ITM2B protein, human; C499960:ITM2C protein, human; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D010750:Phosphoproteins; C530674:integral membrane protein 2b, rat
keywords: Familial British dementia; Familial Danish dementia; Neuritogenesis; PP1 binding motif; PP1 interacting protein; Protein phosphatase 1