Vps4 and the MVB sorting pathway

描述（由申请人提供）：在真核生物中，“多泡体”（MVB）途径将跨膜蛋白和脂质递送到溶酶体内腔中进行降解。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，如营养吸收，细胞通讯和免疫应答都依赖于MVB。MVB分选机器执行独特的膜出芽事件，其导致囊泡形成到内体的内腔中。逆转录病毒如HIV在病毒感染期间协同MVB机制，以通过在质膜处的类似膜出芽事件完成病毒颗粒的形成。因此，MVB分选机已被公认为开发对抗HIV感染的新药的目标，HIV感染是一种每年在全球造成200多万人死亡的疾病。几种多蛋白复合物，称为ESCRT，在MVB执行货物分选和囊泡形成。为了发挥其功能，可溶性ESCRT复合物从细胞质中募集并依次组装在内体膜上，在那里它们将泛素化的货物分选成形成囊泡。为了完成囊泡的形成，ESCRT复合物被AAA型ATP酶Vps4的活性分解。在没有Vps4功能的情况下，ESCRT机制保持在膜上，并且MVB囊泡形成受到抑制。到目前为止，人们一直认为Vps4的拆卸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蛋白质在几个水平上调节Vps4的活性。Vta1似乎增强了刺激Vps4 ATP酶活性的分解反应。Fti1与Vta1共同激活Vps4。Ist 1干扰Vps4功能，Ist 1活性本身似乎受其稳定性调节。基于这些研究结果，我们提出了一个修订的模型，其中Vps4依赖的ESCRT复合物的解体代表了MVB通路内的关键调控步骤。我们认为真核细胞通过调节Ist 1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.