Proteins that leave the cytosol have intrinsic sorting signals that direct them to the appropriate organelle.

The Signal Hypothesis

Sorting signals can be part of these levels of protein structure.

Primary, Secondary, Tertiary, Post-translational modification

Three mechanisms for protein -> organelle delivery

1) Gated Transport - Fully folded proteins pass through a large pore ex. nuclear pore

2) Translocation -  Unfolded proteins are threaded through a translocon (small pore); membrane remains impermeable to small molecules ex. ER, mitochondria, chloroplasts

3) Vesicular Transport - Proteins move between organelles without crossing a membrane while carried by small membrane-bound vesicles ex. secretory and endocytic pathways

A short residue sequence that is recognized by a receptor that delivers the fully-folded peroxisomal protein through a membrane gate/pore.

Peroxisomal Targeting Sequence

This is the only organelle where ribosomes and mRNA are delivered directly.

ER