Regulation of cotranslational N-acetyltransferases by NAC

新合成的蛋白质由各种核糖体相关的生物合成因子进行协同加工，这些因子确保新生多肽的正确成熟、折叠和运输。对于蛋白质的生产性共翻译加工，因子与核糖体结合的时机是非常关键的。例如，在真核生物中，许多胞质底物必须首先被N-末端修饰酶如甲硫氨酸氨基肽酶（METAP）和N-乙酰基转移酶（NAT）加工，然后新生链的折叠被折叠分子伴侣如RAC引发。新生链加工因子如何以底物特异性和时间协调的方式与核糖体结合尚不清楚。在以前的研究中，我们发现，核糖体访问的共翻译因子，运输分泌蛋白（SRP和Sec 61）是由新生的多肽相关复合物（NAC），一个无处不在的异源二聚体复合物位于出口处的核糖体隧道严格调节。最近，我们发现，NAC也专门招募蛋氨酸氨肽酶METAP 1核糖体翻译胞质底物。这些发现表明，NAC也控制初始的共翻译生物合成步骤的胞质蛋白进行的N-末端修饰酶，包括NAT。NAT蛋白家族的五个成员如何特异性地进入新生底物以及它们如何在核糖体隧道出口处与NAC相互作用的机制尚不清楚。为了更好地理解NAT的核糖体相互作用机制，我们将（i）研究NAC在体内N-末端乙酰化的功能重要性，以及（ii）阐明NAT的核糖体结合机制及其与NAC的体外分子相互作用。

Newly synthesized proteins are cotranslationally processed by various ribosome-associated biogenesis factors that ensure proper maturation, folding, and transport of nascent polypeptides. For productive cotranslational processing of proteins, the timing at which a factor binds to ribosomes is very crucial. For example, in eukaryotes, many cytosolic substrates must first be processed by N-terminal modifying enzymes such as methionine aminopeptidases (METAPs) and N-acetyltransferases (NATs) before folding of the nascent chain is initiated by a folding chaperone such as RAC. How the nascent chain processing factors bind to the ribosome in a substrate-specific and temporally coordinated manner is poorly understood. In previous studies, we found that the ribosomal access of cotranslational factors that transport secretory proteins (SRP and Sec61) is tightly regulated by the nascent polypeptide-associated complex (NAC), a ubiquitous heterodimeric complex located at the exit of the ribosomal tunnel. Recently, we discovered that NAC also specifically recruits the methionine aminopeptidase METAP1 to ribosomes translating cytosolic substrates. These findings suggest that NAC also controls initial cotranslational biogenesis steps of cytosolic proteins performed by N-terminal modifying enzymes including NATs. The mechanism of how the five members of the NAT protein family specifically access nascent substrates and how they interact with NAC at the ribosomal tunnel exit is unknown. To better understand the ribosome interaction mechanisms of NATs we will (i) study the functional importance of NAC for N-terminal acetylation in vivo, and (ii) elucidating the ribosome binding mechanisms of NATs and their molecular interplay with NAC in vitro.