| Sažetak | Kompleks mTORC1 pokazao se važnim dijelom signalnih puteva u stanici. Nakon njegovog otkrića, otvorena su brojna pitanja o funkciji ovog kompleksa. Dokazano je da ima brojne zadaće u organizmu vezane uz regulaciju sinteze proteina, biogenezu ribosoma, unos nutrijenata i autofagiju, anablizam aminokiselina te energetski status stanice. Blagi naglasak je stavljen na interakciju kompleksa mTORC1 i leucil-tRNA-sintetaze za koju se pretpostavlja da omogućuje osjećanje aminokiselinskog statusa u citoplazmi stanice, odnosno okolišnu dostupnost nutrijenata. Uz općepoznatu funkciju LRS koja obuhvaća vezanje leucina na odgovarajuću tRNA, otkriveno je da LRS ima važnu funkciju u regulaciji unutarstaničnih procesa koju joj omogućuje interakcija s kompleksom mTORC1. Ključnu ulogu ovdje ima leucin, čije je vezanje za LRS ključno za aktivaciju mTORC1. LRS veže leucin u svoje aktivno mjesto i tada djeluje kao GAP za RagD GTP-azu. Aktivni oblik Rag heterodimera osigurava interakciju heterodimera s kompleksom mTORC1 čime se usmjerava relokalizacija mTORC1 na lizosomalnu membranu. Još jedan način aktivacije mTORC1 kompleksa je putem vakuolarne ATP-aze – ali ne u citoplazmi već na površini lizosoma. Predlaže se inside-out mehanizam prema kojem se aminokiseline akumuliraju unutar lizosoma, a v-ATP-aza ih osjeća na još nepoznat način. v- ATP-aza kontrolira vezanje Rag GTP-aze i Ragulatora, a time i GEF aktivnost Ragulatora. GEF aktivnost Ragulatora očituje se prema proteinima RagA i RagB jer je njihovo GTP vezno stanje važno za aktivaciju kompleksa mTORC1. Aktivirani Rag kompleks veže se za mTORC1 i regrutira ga na lizosomalnu membranu. Pretpostavlja se da se mTORC1 smješta blizu Rheb, jer Rheb-GTP stimulira mTORC1. mTORC1 u interakciji je i s kinazom MAP4K3, koja pod utjecajem određenih induktora aktivira mTORC1 koji će onda aktivirati S6K kinazu,a ona olakšava regrutaciju 40S podjedinice u aktivne polisome čime se poboljšava translacija. Također, postoji i PI3K/Akt signalni put u kojem mTORC1 uzrokuje fosforiliranje 4E-BP1, a on u fosforiliranom stanju napušta EIF-4E, čime se omogućava inicijacija translacije. Oba signalna puta imaju isti cilj – potaknuti translaciju i omogućiti nastajanje proteina koji vode u stanični rast. Uz aktivatore, prisutni su i inhibitori. Po jednom od inhibitora mTORC1 nosi i ime - mehanička meta rapamicina. Zahvaljujući rapamicinu otkriven je mTOR i njegov signalni put. Novija istraživanja ukazuju na to da rapamicin ne inhibira u potpunosti mTORC1, ali se još uvijek ne znaju svi dijelovi metaboličkog puta koje rapamicin inhibira, odnosno ne inhibira. Ursolična kiselina je jedan od inhibitora čija je meta otkrivena – priječi lizosomalnu lokalizaciju mTORC1 kompleksa jer djeluje na Rag GTP-aze. Inhibitori mTORC1 koriste se i u terapijama stanica koje nekontrolirano rastu – tumorima, pa u tom pogledu imaju i korisne značajke. Autofagija je još jedan od procesa koji je koordiniran mTORC1 signalnim putevima obzirom da promovira kataboličke procese, što je suprotno od mTORC1 zadaće – anabolija proteina. Brojne karakteristike mTORC1 interakcija još su nepoznanica. Na primjer, još se ne zna koji točno kompleks i na koji način osjeća razinu aminokiselina, ali mnogo se znanstvenika u svijetu bavi ovom temom i činjenica je da je mTORC1, bez obzira da brojne nepoznanice, još uvijek bolje istražen od mTORC2 kompleksa. U ovom seminarskom radu nisu obuhvaćene sve funkcije i interakcije mTORC1 kompleksa jer ih ima mnogo. Procesi koje mTORC1 posredno ili neposredno regulira prilično su kompleksni i ne čudi da stupa u interakciju s više proteina nego što je u ovom radu navedeno. |
| Sažetak (engleski) | mTORC1 complex is a very important part of signal pathways in the cell. Its discovery posed a variety of questions about its function. It is shown that mTORC1 complex has many duties in protein synthesis regulation, rybosome biogenesis, nutrient intake and autophagy, amino acids anabolism and cell energy status. A mild emphasis was put on mTORC1-LRS interaction for which is thought that enables amino acid presence sensing, especially leucine, in the cell cytoplasm. Except well known LRS leucilation function, it is discovered that LRS also has a major role in intracelular processes guided by mTORC1. Leucine has a key role in this signal pathway. When LRS binds Leu, LRS acts as a GAP for RagD-GTPase. Active form of Rag heterodimer ensures its interaction with mTORC1, which enables mTORC1 relocalization to the lysosome membrane. Another way of mTORC1 activation is by vacuolar ATPase. It is not established in the cytoplasm, but on the lysosome membrane. It is considered that this activation follows an „inside-out“ model by which amino acids are acumulated inside of lysosome, and v-ATPase is sensing them by a still unknown mechanism. v-ATPase controls interaction of Ragulator and Rag GTPase, and by that also the GEF function of Ragulator. Ragulator shows GEF
activity for RagA and RagB proteins, since their GTP bound status is important for mTORC1 activation. Activated Rag complex binds to mTORC1 and recruits it to the lysosome membrane. It is assumed that mTORC1 is located near Rheb, because Rheb-GTP also stimulates mTORC1 activity. mTORC1 interacts with MAP4K3 kinase, which is activated by certain inductors and then can activate mTORC1 complex. Activated mTORC1 then activates S6K kinase which facilitates 40S subunit recruitment into active polysomes. There is also a PI3K/Akt pathway in which mTORC1 ensures phosphorilation of 4E-BP1, which then leaves EIF-4E, and by that, initiation of translation can occur. Both of the signal pathways have the same goal – to induce translation and enable protein synthesis which leads to cell growth. Besides activators there are also inhibitors of mTORC1. mTORC1 is named after one of its inhibitors - mammalian target of rapamycin. Thanks to rapamycin, mTORC1 was disovered. Recent studies indicate that rapamycin does not inhibit all of mTORC1 functions, but it is still unknown which of the pathways rapamycin inhibits. Ursolic acid is one of the inhibitors whose target is known – it inhibits lysosomal translocation of mTORC1 because UA interacts with Rag GTPases. Inhibitors of mTORC1 are used in tumor therapy, so they also have useful applications. Autofagy is another process which is coordinated by mTORC1 signal pathways, considering the fact that it promotes catabolic processes, which is opposite of mTORC1 duties – protein anaboly. Lot of characteristics of mTORC1 that are still unrevealed. For example, it is still not known which protein exactly senses amino acids, but there are lot of scientists interested in this theme and are actively researching. But, regardless of many unknowns, mTORC1 functions are more thoroughly researched than mTORC2. In this seminar not all of the mTORC1 functions and interactions were mentioned because there are too many. Processes that mTORC1 regulates, directly or indirectly, are very complex, therefore it is no surprise that it interacts with more proteins than it was described in this seminar. |
