Cubilin (CUBN) and amnionless (AMN), expressed in kidney and intestine, form a multiligand receptor complex called CUBAM that plays a crucial role in albumin absorption. To date, the mechanism of albumin endocytosis mediated by CUBAM remains to be elucidated. Here, we describe a quantitative assay to evaluate albumin uptake by CUBAM using cells expressing full-length CUBN and elucidate the crucial roles of the C-terminal part of CUBN and the endocytosis signal motifs of AMN in albumin endocytosis. We also demonstrate that nuclear valosin-containing protein-like 2 (NVL2), an interacting protein of AMN, is involved in this process. Although NVL2 was mainly localized in the nucleolus in cells without AMN expression, it was translocated to the extranuclear compartment when coexpressed with AMN. NVL2 knockdown significantly impaired internalization of the CUBN-albumin complex in cultured cells, demonstrating an involvement of NVL2 in endocytic regulation. These findings uncover a link between membrane and nucleolar proteins that is involved in endocytic processes.

Cubilin (CUBN) 和 amnionless (AMN) 在肾脏和肠道中表达，形成称为 CUBAM 的多配体受体复合物，在白蛋白吸收中发挥着至关重要的作用。迄今为止，CUBAM介导的白蛋白内吞作用的机制仍有待阐明。在这里，我们描述了一种使用表达全长 CUBN 的细胞来评估 CUBAM 对白蛋白摄取的定量测定，并阐明了 CUBN C 末端部分和 AMN 的内吞信号基序在白蛋白内吞作用中的关键作用。我们还证明，AMN 的一种相互作用蛋白——核含缬氨肽样蛋白 2 (NVL2) 也参与了这一过程。尽管在不表达AMN的细胞中NVL2主要定位于核仁，但当与AMN共表达时，NVL2易位至核外区室。NVL2 敲除显着损害培养细胞中 CUBN-白蛋白复合物的内化，表明 NVL2 参与内吞调节。这些发现揭示了参与内吞过程的膜蛋白和核仁蛋白之间的联系。