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输送到溶酶体的管腔中进行降解。因此，MVBs对于调节细胞表面蛋白组成和维持溶酶体功能至关重要。许多细胞功能，如营养摄取、细胞通讯和免疫应答都依赖于MVBs。MVB分选机制执行一个独特的膜出芽事件，这导致囊泡的形成进入核内体的管腔。逆转录病毒（如HIV）在病毒感染期间利用MVB机制，通过在质膜上发生类似的膜出芽事件来完成病毒颗粒的形成。因此，MVB分选机器已被公认为开发抗艾滋病毒感染新药的目标，这种疾病每年在全世界造成200多万人死亡。几种称为ESCRTs的多蛋白复合物在MVB中执行货物分选和囊泡形成。为了发挥其功能，可溶性ESCRT复合物从细胞质中招募并依次组装在内体膜上，在那里它们将泛素化的货物分类形成囊泡。为了完成囊泡的形成，ESCRT复合体被aaa型atp酶Vps4的活性分解。没有Vps4功能，ESCRT机制留在膜上，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在多个水平上调节Vps4的活性。Vta1似乎增强了分解反应，刺激Vps4 atp酶活性。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调节。基于这些发现，我们提出了一个修正模型，其中vps4依赖性ESCRT复合物的分解代表了MVB通路中的关键调控步骤。我们提出真核细胞通过调节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.