From the Department of Medicine, University of California San Diego, La Jolla, California 92093,
From the Department of Medicine, University of California San Diego, La Jolla, California 92093.
J Biol Chem. 2018 Oct 5;293(40):15678-15690. doi: 10.1074/jbc.RA118.003080. Epub 2018 Aug 22.
Protein trafficking in the endosomal system involves the recognition of specific signals within the cytoplasmic domains (CDs) of transmembrane proteins by clathrin adaptors. One such signal is the phosphoserine acidic cluster (PSAC), the prototype of which is in the endoprotease furin. How PSACs are recognized by clathrin adaptors has been controversial. We reported previously that HIV-1 Vpu, which modulates cellular immunoreceptors, contains a PSAC that binds to the μ subunits of clathrin adaptor protein (AP) complexes. Here, we show that the CD of furin binds the μ subunits of AP-1 and AP-2 in a phosphorylation-dependent manner. Moreover, we identify a potential PSAC in a cytoplasmic loop of the cellular transmembrane Serinc3, an inhibitor of the infectivity of retroviruses. The two serines within the PSAC of Serinc3 are phosphorylated by casein kinase II and mediate interaction with the μ subunits The sites of these serines vary among mammals in a manner suggesting host-pathogen conflict, yet the Serinc3 PSAC seems dispensable for anti-HIV activity and for counteraction by HIV-1 Nef. The CDs of Vpu and furin and the PSAC-containing loop of Serinc3 each bind the μ subunit of AP-2 (μ2) with similar affinities, but they appear to utilize different basic regions on μ2. The Serinc3 loop requires a region previously reported to bind the acidic plasma membrane lipid phosphatidylinositol 4,5-bisphosphate. These data suggest that the PSACs within different proteins recognize different basic regions on the μ surface, providing the potential to inhibit the activity of viral proteins without necessarily affecting cellular protein trafficking.
蛋白在内体系统中的运输涉及到网格蛋白衔接蛋白对内质网蛋白酶furin 的跨膜蛋白胞质域（CD）中特定信号的识别。这种信号之一是磷酸丝氨酸酸性簇（PSAC），其原型存在于内切蛋白酶furin 中。PSAC 如何被网格蛋白衔接蛋白识别一直存在争议。我们之前报道过，调节细胞免疫受体的 HIV-1 Vpu 含有一个 PSAC，可与网格蛋白衔接蛋白（AP）复合物的μ亚基结合。在这里，我们表明 furin 的 CD 以磷酸化依赖的方式与 AP-1 和 AP-2 的 μ 亚基结合。此外，我们鉴定出细胞跨膜 Serinc3 的胞质环中的一个潜在 PSAC，Serinc3 是逆转录病毒感染性的抑制剂。Serinc3 PSAC 中的两个丝氨酸被酪蛋白激酶 II 磷酸化，并介导与 μ 亚基的相互作用。这些丝氨酸的位点在哺乳动物中因宿主-病原体冲突而变化，但 Serinc3 PSAC 似乎对抗 HIV 活性和 HIV-1 Nef 的拮抗作用是可有可无的。Vpu 和 furin 的 CDs 和 Serinc3 含 PSAC 的环都以相似的亲和力与 AP-2 的 μ 亚基（μ2）结合，但它们似乎利用 μ2 上不同的碱性区域。Serinc3 环需要以前报道的与酸性质膜脂质磷脂酰肌醇 4,5-二磷酸结合的区域。这些数据表明，不同蛋白质中的 PSAC 识别 μ 表面上的不同碱性区域，为抑制病毒蛋白的活性提供了潜力，而不必影响细胞蛋白运输。
