Martin Lucas, Fluhrer Regina, Reiss Karina, Kremmer Elisabeth, Saftig Paul, Haass Christian
Center for Integrated Protein Science Munich and the Adolf Butenandt Institute, Department of Biochemistry, Laboratory for Neurodegenerative Disease Research, Ludwig Maximilians University, 80336 Munich, Germany.
Biochemical Institute, University of Kiel, 24098 Kiel, Germany.
J Biol Chem. 2008 Jan 18;283(3):1644-1652. doi: 10.1074/jbc.M706661200. Epub 2007 Oct 25.
Presenilin, the catalytic component of the gamma-secretase complex, type IV prepilin peptidases, and signal peptide peptidase (SPP) are the founding members of the family of intramembrane-cleaving GXGD aspartyl proteases. SPP-like (SPPL) proteases, such as SPPL2a, SPPL2b, SPPL2c, and SPPL3, also belong to the GXGD family. In contrast to gamma-secretase, for which numerous substrates have been identified, very few in vivo substrates are known for SPP and SPPLs. Here we demonstrate that Bri2 (Itm2b), a type II-oriented transmembrane protein associated with familial British and Danish dementia, undergoes regulated intramembrane proteolysis. In addition to the previously described ectodomain processing by furin and related proteases, we now describe that the Bri2 protein, similar to gamma-secretase substrates, undergoes an additional cleavage by ADAM10 in its ectodomain. This cleavage releases a soluble variant of Bri2, the BRICHOS domain, which is secreted into the extracellular space. Upon this shedding event, a membrane-bound Bri2 N-terminal fragment remains, which undergoes intramembrane proteolysis to produce an intracellular domain as well as a secreted low molecular weight C-terminal peptide. By expressing all known SPP/SPPL family members as well as their loss of function variants, we demonstrate that selectively SPPL2a and SPPL2b mediate the intramembrane cleavage, whereas neither SPP nor SPPL3 is capable of processing the Bri2 N-terminal fragment.
早老素是γ-分泌酶复合物的催化成分，IV型前导肽肽酶和信号肽肽酶（SPP）是膜内裂解的GXGD天冬氨酸蛋白酶家族的创始成员。SPP样（SPPL）蛋白酶，如SPPL2a、SPPL2b、SPPL2c和SPPL3，也属于GXGD家族。与已鉴定出众多底物的γ-分泌酶不同，已知SPP和SPPLs的体内底物很少。在此，我们证明与家族性英国和丹麦痴呆症相关的II型跨膜蛋白Bri2（Itm2b）经历了受调控的膜内蛋白水解。除了先前描述的由弗林蛋白酶和相关蛋白酶进行的胞外域加工外，我们现在描述Bri2蛋白与γ-分泌酶底物类似，在其胞外域还会被ADAM10进一步切割。这种切割释放出Bri2的可溶性变体BRICHOS结构域，其被分泌到细胞外空间。在这种脱落事件发生后，一个膜结合的Bri2 N端片段保留下来，该片段会经历膜内蛋白水解以产生一个细胞内结构域以及一个分泌的低分子量C端肽。通过表达所有已知的SPP/SPPL家族成员及其功能缺失变体，我们证明选择性地SPPL2a和SPPL2b介导膜内切割，而SPP和SPPL3都不能加工Bri2 N端片段。
