RESEARCHCOMMUNICATIONPOT1-interactingproteinPIP1:atelomerelengthregulatorthatrecruitsPOT1totheTIN2/TRF1complexJeffreyZheng-ShengYe,1,3,4,5DirkHockemeyer,1AndrewN.Krutchinsky,2DiegoLoayza,1SarahM.Hooper,1BrianT.Chait,2andTitiadeLange1,61LaboratoryforCellBiologyandGeneticsand2LaboratoryforMassSpectrometryandGaseousIonChemistry,TheRockefellerUniversity,NewYork,NewYork10021,USA;3DepartmentofMedicine,MemorialSloan-KetteringCancerCenter,NewYork,NewYork10021,USAHumantelomerelengthiscontrolledbyanegativefeed-backloopbasedonthebindingofTRF1todouble-strandedtelomericDNA.TheTRF1complexrecruitsPOT1,asingle-strandedtelomericDNA-bindingproteinnecessaryforcis-inhibitionoftelomerase.Bymassspec-trometry,wehaveidentifiedanewtelomericprotein,whichwehavenamedPOT1-interactingprotein1(PIP1).PIP1boundbothPOT1andtheTRF1-interactingfactorTIN2andcouldtetherPOT1totheTRF1com-plex.ReductionofPIP1orPOT1levelswithshRNAsledtotelomereelongation,indicatingthatPIP1contributestotelomerelengthcontrolthroughrecruitmentofPOT1.Supplementalmaterialisavailableathttp://www.genesdev.org.ReceivedApril26,2004;revisedversionacceptedMay14,2004.Intelomerase-positivehumancells,telomerelengthisinfluencedbyaproteincomplex,builtonthetelomericDNA-bindingproteinTRF1(forreview,seeSmogor-zewskaanddeLange2004).TheTRF1complexcontainsaTRF1-interactingprotein,TIN2,andPOT1,aproteinwithanN-terminalOBfoldthatconfersbindingtosingle-strandedTTAGGGrepeats(Chongetal.1995;Kimetal.1999;BaumannandCech2001;LoayzaanddeLange2003;Loayzaetal.2004).TheamountsofTRF1,TIN2,andPOT1areincreasedonlongertelomeres(Smogorzewskaetal.2000;LoayzaanddeLange2003),consistentwitha“protein-counting”modeloftelomerelengthcontrol(Marcandetal.1997).WhenTRF1orTIN2isinhibitedwithmutantallelesorRNAi,telo-meresbecomeoverelongatedbytelomerase,indicatingthatcellsmeasuretelomerelengthbasedontheamountofTRF1complexpresent(vanSteenselanddeLange1997;Kimetal.1999;YeanddeLange2004).APOT1mutant(POT1�OB)thatisincapableofbindingtosingle-strandedtelomericDNAbecauseofdeleti...
