B e f or e w e s ee how s om e pr ot e i n’ s s ynt he s i s i s di r ec t e d t o t he E R , l e t ’ s l ook a t a n

ove r vi e w of p r ot e i n t a r ge ti ng.

P r ot e i n t arge t i n g r e f e r s t o ge tti ng a pr ot e i n w he r e it ne e ds t o be i n t he ce ll .

T h e s yn t h e s i s of al l nu c l e ar - e n c od e d p r ot e i n s S T AR T on f ree r i b os om e s i n t h e

c yt op l as m . F i g u re 12.15, p age 272.

O nc e t r a ns l a ti on be gi ns t he a m i no ac i ds o f t he pr ot e i n t ha t i s be i ng m a de be gi n t o e m e r ge

fr om t he r i bos om e . N ow a de c i s i on a s t o be m a de a bout w he r e t o C O M P L ETE t he

s ynt he s i s of pr ot e i ns .

T he r e a r e t w o ca t e gor i e s of pr ot e i ns . S om e p r ot e i n s a r e c om pl e t e d on fr e e r i bos om e s , a nd

( 2) s om e p r ot e i ns have t he i r pr ot e i n s ynt he s i s c om pl e t e d on r i bos om e s a tt ac he d t o t he

E R .

H ow i s t he de c i s i on m a de ? T he i n f or m a ti on i s gi v e n a t t he be gi nni ng of t he ne w l y

s ynt he s i z e d pol ype pt i de , a nd i s ba s e d on t he a m i n o ac i d s e que nc e .

P r ot e i n s w i t h o u t a s i gn al p e p t i d e c om p l e t e t r an s l at i on on f r ee r i b os om e s .

F i gur e 12. 15 , page 272, l e f t s i de .

T he t ype o f pr o t e i ns t ha t do not have s i gna l pe pt i d e s a r e pr ot e i ns t ha t a r e de s ti ne d f or t he

doubl e m e m br a ne or gane ll e s ( nuc l e us , c hl or opl a s t , or m it oc hondr i a ) or pr ot e i ns of t he

c yt os ol , o r t o a n o r gane ll e know n a s a pe r oxi s om e . A ll of t he s e t ype s of pr ot e i ns have

t he i r t r a ns l a ti on be gun a nd c om pl e t e d on fr ee r i bos om e s (r i bos om e s i n t he c yt os ol ) .

O nc e s uc h pol ype pt i de s a r e c r ea t e d, t he n t he s e que nc e of ce r t a i n a m i no ac i ds w it hi n t he

pol y pe pt i de di r ec t s t he m t o t he i r pr ope r or gane ll e a nd t he y m ove t he r e .

F or e xa m pl e nuc l ea r pr ot e i ns have a nuc l ea r t a r ge t s e que nc e t ha t a ll ow s t he m t o ge t

t hr ough t he nuc l ea r por e s , a nd i nt o t he nuc l e us .

M it oc hondr i a l pr ot e i ns have a l oc a li za ti on s i gna l t ha t ge t s t he m i nt o t he m it oc hond r i a ,

a nd pl a s ti d pr ot e i ns have a l oc a li za ti on s i gna l t ha t ge t s t he m i nt o t he pl a s ti d.

If a p ol ype pt i de i s m a de on a f r ee r i bos om e a nd ha s ne it he r a nuc l ea r , m it oc hondr i a l ,

pe r oxi s om a l or pl a s ti d l oc a li za ti on s i gna l i t w ill e nd up i n t he li qui d por ti on of t he

c yt opl a s m ( t he li qui d pa r t o f t he c yt opl a s m i s kno w n a s t he c yt os ol ) .

N ow l e t’ s l ook at p r ot e i n s t h at c om p l e t e t r an s l a t i on on r i b os om e s at t ac h e d t o t h e

E R .

M a ny pr ot e i ns have t he i r t r a ns l a ti on S T A R T on fr ee r i bos o m e s but t he i r t r a ns l a ti on w ill

be C O M P LET E D on r i bos om e s t ha t a r e a tt ac he d t o t he e ndopl a s m i c r e ti c ul um .

T he s e a r e pr ot e i ns t ha t have a s e que nc e ne a r t he a m i no t e r m i nus o f t he pol ype pt i de