!The!Cell!Cycle!(This&topic&is&intense!)&!Gene+cally!encoded!fluorescent!probes!label!individual!G1!phase!nuclei!red!and!those!in!S/G2/M!phases!green.!Asako&Sakaue-Sawano&et&al.,&2008,&Cell.&&Recent!Classes:!!Interphase!&!M-phase&Figure 17-4 Molecular Biology of the Cell (© Garland Science 2008) The&cell!cycle!is&the&ordered&series&of&events&that&leads&to&cell&duplicaAon&and&division.!M-phase!occupies&a&small&fracAon&of&the&cell&cycle.&The&rest&of&the&cell&cycle&is&called&interphase&and&consists&of&three&phases:&&G1-phase!(Gap&1)&is&the&period&between&the&compleAon&of&the&previous&mitosis&and&the&iniAaAon&of&DNA&synthesis&for&the&next&mitosis.&&S-phase!is&the&period&of&DNA&synthesis&for&chromosome&duplicaAon.&&G2-phase!(Gap&2)&is&the&period&between&the&compleAon&of&DNA&replicaAon&and&the&iniAaAon&of&mitosis.&(Some&cells&exit&the&cell&cycle&in&G1.&These&post-mitoAc,&non-proliferaAng&(quiescent)&cells&are&said&to&be&in&G0!(G-zero).)&G0&Figure 17-14 Molecular Biology of the Cell (© Garland Science 2008) The!cell!cycle!is!+ghtly!regulated!&Progression&through&the&cell&cycle&relies&on&cell&growth&and&successful&passage&through&several&checkpoints&that&ensure&the&accuracy&and&fidelity&of&chromosome&replicaAon.&Chromosome&replicaAon&and&cell&division&must&occur&in&the&proper&order&and&proper&Ame&in&every&cycle.&In&many&cells,&growth&and&division&rates&are&coupled&(although&the&two&processes&can&actually&be&regulated&independently&of&each&other).&Cell&size&is&determined&by&the&rate&of&growth&and&the&duraAon&of&the&cell&cycle.&&&In&animal&cells,&both&cell&growth&and&cell&division&depend&on&extracellular&signals.&&Signaling&during&the&cell&cycle&relies&on&post-transla+onal!modifica+ons!such&as&(1)&phosphoryla+on&of&tyrosines,&serines,&and&threonines,&and&(2)&ubiqui+na+on&on&lysines.&&PhosphorylaAon&is&reversible.&UbiquiAnaAon&oRen&triggers&protein&degradaAon,&an&irreversible&acAon.&G0&In&response&to&growth&factors,&receptor!tyrosine!kinases!(RTKs)!recruit&and&acAvate&PI!3-kinases&(PI3K)!at&the&plasma&membrane.&PI3K&is&a&lipid&kinase&that&binds&to&phospho-tyrosine&residues&in&the&cytoplasmic&domains&of&acAvated&RTKs,&which&brings&PI3K&in&proximity&to&phospholipids&on&the&cytosolic&face&of&the&membrane.&PI3K&phosphorylates&the&3’&posiAon&of&inositols&to&generate&PI(3,4,5)P3&and&other&phosphaAdylinosiAdes.&Cell!growth!is!driven!by!PI!3-kinase!pathways&AccumulaAon&of&PI(3,4,5)P3&in&the&membrane&leads&to&acAvaAon&of&a&serine/threonine&kinase&called&Akt.&Once&acAvated,&Akt&phosphorylates&many&proteins&in&the&cytoplasm&and&nucleus.&One&target&that&promotes&cell&growth&is&another&protein&kinase&called&mTOR.&AcAvaAon&of&mulA-subunit&complexes&containing&mTOR&results&in&phosphorylaAon&of&targets&involved&in&protein&synthesis&and&metabolism.&One&criAcal&substrate&of&mTOR&is&S6-kinase,&which&phosphorylates&ribosomal&protein&S6,&resulAng&in&increased&translaAon&of&mRNAs.&A&second&criAcal&substrate&is&4E-BP,&an&inhibitor&of&the&translaAon&iniAaAon&factor&elF4E.&PhosphorylaAon&of&4E-BP&releases&elF4E&from&4E-BP&inhibiAon&and&allows&it&to&iniAate&translaAon.&Figure 17-65 Molecular Biology of the Cell Akt!!Two&major&protein&degradaAon&pathways&exist&in&eukaryoAc&cells:&Lysosome&pathways&and&Proteasome&pathways.&(We&previously&discussed&lysosomal&degradaAon,&and&we&will&re-visit&that&topic&when&we&discuss&autophagy&and&cell&death.)&&The&proteasome&is&very&large&macromolecular&machine&consisAng&of&~50&protein&subunits&that&act&collecAvely&to&degrade&many&cellular&proteins.&&A&typical&mammalian&cell&contains&~30,000&proteasomes.&The&proteasome&hydrolyzes&ATP&to&provide&the&energy&needed&for&degradaAon.&The&life&span&of&intracellular&proteins&varies&from&a&few&minutes&to&as&long&as&the&age&of&an&organism.&Protein&life&span&is&primarily&controlled&by&regulated&protein&degradaAon.&&Biosynthe+c!growth!can!be!balanced!by!protein!degrada+on!in!the!proteasome&K.Luo&Three&important&funcAons&of&protein&degradaAon:&1.&It&removes&proteins&that&are&misfolded,&damaged,&or&potenAally&toxic&to&the&cell.&2.&Controlled&degradaAon&of&normal&proteins&provides&a&powerful&mechanism&to&maintain&appropriate&levels&of&proteins&and&their&acAviAes.&3.&Regulated&degradaAon&also&permits&rapid&responses&to&changing&condiAons.&How&do&the&proteasomes&disAnguish&proteins&that&are&to&be&degraded&from&normal&ones?&&&Ubiqui+na+on!&Ubiqui+n&is&&a&highly&conserved&polypepAde&of&76&aa&that&marks&proteins&for&degradaAon&by&the&proteasome.&MulAple&ubiquiAn&molecules&can&be&agached&covalently&to&proteins&via&a&mulA-step&process&called&poly-ubiquiAnaAon.&Proteins&that&are&poly-ubiqui+nated!are&recognized&by&the&proteasome.&The&ubiquiAnaAon&process&involves&several&enzymes,&and&the&specificity&of&poly-ubiquiAnaAon&is&achieved&through&E3!Ub-ligases.&Mammalian&cells&express&many&E3&ligases,&and&each&recognizes&a&specific&signal&in&a&given&protein&for&poly-ubiquiAnaAon.&&(The&Nobel&Prize&for&Chemistry&in&2004&was&awarded&for&the&discovery&of&ubiquiAn-mediated&protein&degradaAon.)&Ubiqui+n!marks!proteins!for!degrada+on&K.Luo&Cell!cycle!progression!is!controlled!by!protein!expression,!phosphoryla+on,!&!degrada+on&Passage&through&the&cell&cycle&is&controlled&by&heterodimeric&protein&complexes&consisAng&of&a&regulatory&subunit&(a&cyclin)&and&a&catalyAc&subunit&(a&cyclin-dependent!kinase!–&CDK).&The&CDKs&depend&on&cyclins&for&acAvaAon&of&their&kinase&acAvity&and&for&access&to&substrates.&Mammalian&cells&have&mulAple&cyclin-CDK&complexes&that&are&expressed&and&funcAon&at&different&phases&of&cell&cycle:&Early&G1:&cyclin&D-CDK4,6&Late&G1/S:&cyclin&E-CDK2&S:&cyclin&A-CDK2&M:&cyclin&A,B-CDK1&&Together,!cyclin/CDK!complexes,!protein&phosphatases!and!ubiqui+n!ligases!regulate!cell!cycle !transi+ons.&G1/S!cyclin-CDKs!G1!cyclin-CDKs!Figure 17-16 Molecular Biology of the Cell (© Garland Science 2008) The!synthesis!and!degrada+on!of!cyclins!control!progression!through!the!cell!cycle!&The!concentra+ons!of!the!cyclin!proteins!oscillate!during!the!cell!cycle!while!the!concentra+ons!of!the!Cdks!remain!steady&Cdks!1.&AcAvaAon&by&cyclin-binding&and&T-loop&phosphorylaAon&–&Binding&of&a&cyclin&to&a&CDK&alters&the&conformaAon&of&an&area&called&the&T-loop,&leading&to&a&parAal&acAvaAon&of&the&CDK.&PhosphorylaAon&by&a&CDK-ac+va+ng!kinase!(CAK)!of&a&specific&T-loop&residue&induces&an&addiAonal&conformaAonal&change&that&allows&substrate&access&and&full&CDK&acAvaAon.&CDK!ac+vity!is!+ghtly!regulated&Figure 17-17 Molecular Biology of the Cell (© Garland Science