The main organelles in protein sorting and targeting are Rough endoplasmic reticulum. As the question is all about sorting and targeting to membrane and secretion, I will stick to only these two targeting pathways. Please note the mechansm of targeting proteins to other organelles such as mitichondria, ER, chloroplast is different. Journey of a secretory protein: The protein sorting and targeting occur in endoplasmic reticulum. Most of the secretory proteins as well as membrane proteins are translocated into ER co-translationally (Co-translational translocation). i.e, they are moved into ER while their translation is going on. this translocation process is mediated by Signal recognition particle (SRP). SRP recognizes N-terminal signal peptide on the protein that is being translated. The translation process pauses for a while when the ribosome-protein-SRP complex is being transported to SRP receptor on the ER membrane. The protein is inserted into translocon channel on ER membrane and enters ER. The signaal sequence from nascent peptide is immediately cleaved in case of secretoey proteins and type I transmembrane proteins by signal peptidase. the translation resumes directing the protein being formed into ER. Inside the ER, chaperone proteins bind the protein to guide through correct folding. Then the protein is transported by ER membrane vesicles to golgi apparatus for further processing like protein modification and glycosylation. ER--- cis-golgi---trans---golgi--- lysosomes. the proteins from lysosomes fuse with membrane and according to the signals present in nascent proteins, they either remain as transmembrane proteins or secreted out of the membrane. transmembrane G-receptors span the membrane seven times (also called serpentine receptors) donot contain signal sequence at N-terminal. The first transmembrane domain acts as the signal sequence that is recognized by SRP. Some secretory proteins as well as transmembrane proteins may undero post-translational translocation. i.e, transportation to ER after the completion of protein synthesis. the entire process is believed to be same as co-translational translocation. Solution The main organelles in protein sorting and targeting are Rough endoplasmic reticulum. As the question is all about sorting and targeting to membrane and secretion, I will stick to only these two targeting pathways. Please note the mechansm of targeting proteins to other organelles such as mitichondria, ER, chloroplast is different. Journey of a secretory protein: The protein sorting and targeting occur in endoplasmic reticulum. Most of the secretory proteins as well as membrane proteins are translocated into ER co-translationally (Co-translational translocation). i.e, they are moved into ER while their translation is going on. this translocation process is mediated by Signal recognition particle (SRP). SRP recognizes N-terminal signal peptide on the protein that is being translated. The translation process pauses for a .

1. The main organelles in protein sorting and targeting are Rough endoplasmic reticulum. As the
question is all about sorting and targeting to membrane and secretion, I will stick to only these
two targeting pathways. Please note the mechansm of targeting proteins to other organelles such
as mitichondria, ER, chloroplast is different.
Journey of a secretory protein:
The protein sorting and targeting occur in endoplasmic reticulum. Most of the secretory proteins
as well as membrane proteins are translocated into ER co-translationally (Co-translational
translocation). i.e, they are moved into ER while their translation is going on. this translocation
process is mediated by Signal recognition particle (SRP). SRP recognizes N-terminal signal
peptide on the protein that is being translated. The translation process pauses for a while when
the ribosome-protein-SRP complex is being transported to SRP receptor on the ER membrane.
The protein is inserted into translocon channel on ER membrane and enters ER. The signaal
sequence from nascent peptide is immediately cleaved in case of secretoey proteins and type I
transmembrane proteins by signal peptidase. the translation resumes directing the protein being
formed into ER. Inside the ER, chaperone proteins bind the protein to guide through correct
folding. Then the protein is transported by ER membrane vesicles to golgi apparatus for further
processing like protein modification and glycosylation. ER--- cis-golgi---trans---golgi---
lysosomes. the proteins from lysosomes fuse with membrane and according to the signals present
in nascent proteins, they either remain as transmembrane proteins or secreted out of the
membrane.
transmembrane G-receptors span the membrane seven times (also called serpentine receptors)
donot contain signal sequence at N-terminal. The first transmembrane domain acts as the signal
sequence that is recognized by SRP.
Some secretory proteins as well as transmembrane proteins may undero post-translational
translocation. i.e, transportation to ER after the completion of protein synthesis. the entire
process is believed to be same as co-translational translocation.
Solution
The main organelles in protein sorting and targeting are Rough endoplasmic reticulum. As the
question is all about sorting and targeting to membrane and secretion, I will stick to only these
two targeting pathways. Please note the mechansm of targeting proteins to other organelles such
as mitichondria, ER, chloroplast is different.
Journey of a secretory protein:
The protein sorting and targeting occur in endoplasmic reticulum. Most of the secretory proteins

2. as well as membrane proteins are translocated into ER co-translationally (Co-translational
translocation). i.e, they are moved into ER while their translation is going on. this translocation
process is mediated by Signal recognition particle (SRP). SRP recognizes N-terminal signal
peptide on the protein that is being translated. The translation process pauses for a while when
the ribosome-protein-SRP complex is being transported to SRP receptor on the ER membrane.
The protein is inserted into translocon channel on ER membrane and enters ER. The signaal
sequence from nascent peptide is immediately cleaved in case of secretoey proteins and type I
transmembrane proteins by signal peptidase. the translation resumes directing the protein being
formed into ER. Inside the ER, chaperone proteins bind the protein to guide through correct
folding. Then the protein is transported by ER membrane vesicles to golgi apparatus for further
processing like protein modification and glycosylation. ER--- cis-golgi---trans---golgi---
lysosomes. the proteins from lysosomes fuse with membrane and according to the signals present
in nascent proteins, they either remain as transmembrane proteins or secreted out of the
membrane.
transmembrane G-receptors span the membrane seven times (also called serpentine receptors)
donot contain signal sequence at N-terminal. The first transmembrane domain acts as the signal
sequence that is recognized by SRP.
Some secretory proteins as well as transmembrane proteins may undero post-translational
translocation. i.e, transportation to ER after the completion of protein synthesis. the entire
process is believed to be same as co-translational translocation.