Crepaldi T, Prat M, Giordano S, Medico E, Comoglio P M
Department of Biomedical Sciences and Oncology, University of Torino Medical School, Italy.
J Biol Chem. 1994 Jan 21;269(3):1750-5.
The hepatocyte growth factor (HGF) receptor (p190MET) is a tyrosine kinase composed of two disulfide-linked chains, alpha of 50 kDa and beta of 145 kDa. We have previously described an isoform (p140MET) containing a beta chain of 85 kDa, lacking the cytoplasmic kinase domain. The two receptor variants originate by post-translational processing of a common single-chain precursor of 170 kDa (Pr170). In the endoplasmic reticulum a fraction of Pr170 is cleaved at the cytosolic side generating an intermediate product of 120 kDa (Pr120). This molecule 1) is already detectable after 15 min of pulse labeling, 2) contains high mannose-branched oligosaccharides, and 3) accumulates upon treatments inhibiting the export from the endoplasmic reticulum. A second cleavage, occurring after 30 min of chase in the trans-Golgi network, converts the single-chain precursors Pr170 and Pr120 into the mature heterodimers p190MET and p140MET. This process is inhibited by brefeldin A treatment. Conditions leading to Pr170 accumulation in the endoplasmic reticulum, such as receptor overexpression, induce kinase activation and overproduction of Pr120. Conversely, cells expressing a kinase-defective HGF receptor lack the truncated isoform. The proteolytic cleavage of the cytoplasmic domain may thus represent a safety mechanism aimed at preventing ligand-independent intracellular activation of the HGF receptor kinase.
肝细胞生长因子（HGF）受体（p190MET）是一种酪氨酸激酶，由两条通过二硫键连接的链组成，α链为50 kDa，β链为145 kDa。我们之前描述过一种异构体（p140MET），其含有一条85 kDa的β链，缺少细胞质激酶结构域。这两种受体变体源自170 kDa的共同单链前体（Pr170）的翻译后加工。在内质网中，一部分Pr170在胞质侧被切割，产生120 kDa的中间产物（Pr120）。该分子1）在脉冲标记15分钟后即可检测到，2）含有高甘露糖型分支寡糖，3）在抑制从内质网输出的处理后会积累。第二次切割发生在反式高尔基体网络中追踪30分钟后，将单链前体Pr170和Pr120转化为成熟的异二聚体p190MET和p140MET。此过程受布雷菲德菌素A处理的抑制。导致Pr170在内质网中积累的条件，如受体过表达，会诱导激酶激活和Pr120的过量产生。相反，表达激酶缺陷型HGF受体的细胞缺乏截短的异构体。因此，细胞质结构域的蛋白水解切割可能代表一种安全机制，旨在防止HGF受体激酶的非配体依赖性细胞内激活。
