Vps4 and the MVB sorting pathway

描述(申请人提供)：在真核生物中，“多囊泡小体”(MVB)途径将跨膜蛋白和脂类输送到溶酶体的管腔中进行降解。因此，MVB对于调节细胞表面蛋白组成和维持溶酶体功能是必不可少的。许多细胞功能，如营养摄取、细胞通讯和免疫反应都依赖于MVB。MVB分拣机械执行独特的膜萌发事件，导致囊泡形成进入内体的管腔。逆转录病毒，如艾滋病毒，在病毒感染期间利用MVB机制，通过质膜上类似的膜萌发事件完成病毒颗粒的形成。因此，MVB分拣机械已被认为是开发抗击艾滋病毒感染的新药的目标，艾滋病毒感染是一种每年在全球范围内导致200多万人死亡的疾病。几个多蛋白复合体，称为ESCRT，在MVB执行货物分拣和囊泡形成。为了执行它们的功能，可溶性ESCRT复合体从细胞质中招募，并顺序地组装在内体膜上，在那里它们将泛素化的货物分类成形成囊泡。为了完成囊泡的形成，ESCRT复合体通过AAA型ATPase Vps4的活性被分解。如果没有Vps4功能，ESCRT机制会留在膜上，MVB囊泡的形成会受到抑制。到目前为止，人们一直认为Vps4的分解功能是一个本构过程。然而，我们的初步研究已经确定了三种在不同水平上调节Vps4活性的蛋白质。Vta1似乎增强了刺激Vps4 ATPase活性的拆解反应。Fti1与Vta1一起激活Vps4。Ist1干扰Vps4功能，Ist1活性本身似乎受其稳定性的调节。基于这些发现，我们提出了一个修正的模型，在该模型中，Vps4依赖的ESCRT复合体的分解代表了MVB途径中的关键调控步骤。我们认为真核细胞通过调节Ist1的降解速率来调节MVB通路的活性，从而调节Vps4的活性。

DESCRIPTION (provided by applicant): In eukaryotes, the 'multivesicular body' (MVB) pathway delivers transmembrane proteins and lipids into the lumen of the lysosome for degradation. As a consequence, MVBs are essential for regulating cell surface protein composition and maintaining lysosomal function. Numerous cellular functions, such as nutrient uptake, cell communication and immune response are dependent on MVBs. The MVB sorting machinery performs a unique membrane budding event, which results in the formation of vesicles into the lumen of the endosome. Retroviruses such as HIV co-opt the MVB machinery during viral infection to complete formation of viral particles via a similar membrane budding event at the plasma membrane. Therefore, the MVB sorting machinery has been recognized as a target for the development of new drugs combating HIV infection, a disease that kills more than 2 million people per year worldwide. Several multiprotein complexes, called the ESCRTs, execute cargo sorting and vesicle formation at the MVB. To perform their function, the soluble ESCRT complexes are recruited from the cytoplasm and sequentially assemble on the endosomal membrane where they sort ubiquitinated cargo into forming vesicles. To complete vesicle formation, the ESCRT complexes are disassembled by the activity of the AAA-type ATPase Vps4. Without Vps4 function, the ESCRT machinery remains on the membrane and MVB vesicle formation is inhibited. Up until now, it had been thought that the disassembly function of Vps4 was a constitutive process. However, our preliminary studies have identified three proteins that regulate the activity of Vps4 on several levels. Vta1 appears to enhance the disassembly reaction that stimulates Vps4 ATPase activity. Fti1 works together with Vta1 in the activation of Vps4. Ist1 interferes with Vps4 function and Ist1 activity itself appears to be regulated by its stability. Based on these findings we propose a revised model in which the Vps4-dependent disassembly of the ESCRT complexes represents a key regulatory step within the MVB pathway. We propose that eukaryotic cells modulate the activity of the MVB pathway by regulating the degradation rate of Ist1, thereby regulating the activity of Vps4.