Protein Sorting and Import Mechanisms in Cells

Translation Site

Determines final protein location in the cell.

Free-Floating Ribosome

Ribosome without signal; remains in cytoplasm.

Co-Translational Sorting

Proteins sent to ER during synthesis.

Post-Translational Sorting

Proteins sorted after complete synthesis.

Nuclear Pore Complex

Allows passive diffusion of small molecules.

Passive Diffusion

Movement without energy for molecules < 20 kDa.

Active Transport

Energy-dependent movement for molecules > 60 kDa.

Nuclear Localization Signal (NLS)

Signal rich in basic amino acids for nuclear import.

Bipartite NLS

Nuclear signal consisting of two separate parts.

Nuclear Export Signal (NES)

Leucine-rich signal keeping proteins out of nucleus.

Importin Protein

Binds NLS and transports proteins into the nucleus.

RanGTP

Binds importin, facilitating cargo release.

Exportin

Recognizes NES for protein export from nucleus.

RanGDP

Form of Ran after GTP hydrolysis; releases exportin.

Mitochondrial Genome

Contains 13 protein-coding genes, 22 tRNA genes.

Mitochondrial Protein Import

Involves TOM and TIM translocators for protein entry.

Chaperonin Complex

Assists in proper protein folding in mitochondria.

Signal Recognition Particle (SRP)

Pauses translation for RER signal binding.

Endomembrane System

Pathway: RER → Golgi → Secretory vesicles → Plasma membrane.