Abbate M, Bachinsky D R, McCluskey R T, Brown D
Renal Unit, Massachusetts General Hospital, Boston.
J Am Soc Nephrol. 1994 Jun;4(12):2003-15. doi: 10.1681/ASN.V4122003.
The gp330/alpha 2-macroglobulin receptor-associated protein (RAP) is a 39- to 45-kd protein that binds to the low-density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor and to gp330, a major glycoprotein of the brush border of proximal tubule cells. Despite evidence that gp330 functions as a receptor for several ligands and that soluble RAP inhibits ligand binding to gp330 in vitro, the physiologic function of RAP is unknown. Given the predominant location of RAP within the rough endoplasmic reticulum (RER), RAP might be involved in the intracellular processing and/or transport of gp330. The developing rat kidney was used as a dynamic model to study in detail the relationship between gp330 and RAP in vivo by immunohistochemical techniques. RAP was expressed in the renal vesicle and continued to be present, with a vesicular and perinuclear pattern of staining, in both proximal tubule cells and glomerular cells at subsequent stages. Immunoperoxidase electron microscopy demonstrated RAP in cisternae of the RER and in large subapical vesicles. gp330 was initially expressed in early proximal tubule cells in S-shaped bodies and was located in the perinuclear envelope and cytoplasmic vesicles as well as at the apical surface. Cytoplasmic gp330 staining was more evident at a stage subsequent to the S-shaped body, possibly related to more active biosynthesis. By comparative analysis of the patterns of immunofluorescence and immunoperoxidase staining, gp330 and RAP colocalized in the RER and in some large subapical vacuoles, but no definite RAP staining could be detected at the surface of proximal tubule cells at any stage, despite the presence of abundant gp330 in this location. The expression of gp330 at the apical surface of immature tubular cells was associated with the onset of fluid-phase endocytosis of fluoroscein isothiocyanate-dextran and, therefore, of reabsorption of material from the tubular lumen, in the absence of concomitant changes in RAP expression in the same cells. These findings indicate that the role of endogenous RAP may not be directly related to ligand binding of gp330 at the surface of proximal tubule cells, although RAP may be involved in the processing and the intracellular trafficking of newly synthesized gp330, in particular in the delivery of gp330 to the plasma membrane.
gp330/α2-巨球蛋白受体相关蛋白（RAP）是一种39至45千道尔顿的蛋白质，它能与低密度脂蛋白受体相关蛋白/α2-巨球蛋白受体以及gp330结合，gp330是近端小管细胞刷状缘的一种主要糖蛋白。尽管有证据表明gp330作为几种配体的受体发挥作用，且可溶性RAP在体外可抑制配体与gp330的结合，但RAP的生理功能尚不清楚。鉴于RAP主要位于粗面内质网（RER）内，它可能参与gp330的细胞内加工和/或运输。发育中的大鼠肾脏被用作动态模型，通过免疫组织化学技术详细研究体内gp330与RAP之间的关系。RAP在肾小泡中表达，并在随后阶段继续存在于近端小管细胞和肾小球细胞中，呈现泡状和核周染色模式。免疫过氧化物酶电子显微镜显示RAP存在于RER的池和大的顶端下小泡中。gp330最初在S形体的早期近端小管细胞中表达，位于核周包膜、细胞质小泡以及顶端表面。细胞质gp330染色在S形体之后的阶段更为明显，可能与更活跃的生物合成有关。通过对免疫荧光和免疫过氧化物酶染色模式的比较分析，gp330和RAP在RER以及一些大的顶端下液泡中共定位，但在近端小管细胞表面在任何阶段都未检测到明确的RAP染色，尽管该位置存在大量gp330。未成熟肾小管细胞顶端表面gp330的表达与异硫氰酸荧光素 - 葡聚糖的液相内吞作用开始相关，因此与从肾小管腔重吸收物质相关，而同一细胞中RAP表达无伴随变化。这些发现表明，内源性RAP的作用可能与近端小管细胞表面gp330的配体结合无直接关系，尽管RAP可能参与新合成的gp330的加工和细胞内运输，特别是将gp330递送至质膜。
