Lecture 24 Protein Translocation, Organellar Genomes
Page 1: Overview
Lecture 24 Topics:
Protein Translocation
Organellar Genomes
Page 2: Learning Goals
Understand how nuclearly-encoded proteins reach their destination:
Cytoplasmic Proteins
Nuclear Proteins
Organellar Proteins
Exported Proteins and Endomembrane System
Identify organelles with their own genomes and the fate of ancestral genomes of mitochondrion and chloroplast.
Page 3: Protein Synthesis Basics
Cytosolic proteins synthesized in the cytosol remain there.
Other proteins require embedding in membranes or compartments, which begins with protein synthesis in cytoplasm.
Nuclearly-encoded proteins cannot pass through the lipid bilayer directly.
Page 4: Membrane Insertion
Some proteins are amphipathic, embedded in membranes.
Mechanism for crossing plasma membrane for exported proteins is discussed.
Most organellar genes are nuclearly encoded; proteins synthesized in cytoplasm must find their way to organelles.
Page 5: Nuclear Protein Transfer
Does nuclear transfer require crossing a membrane?
Yes, through the nuclear membrane (both inner and outer).
Nuclear pores allow diffusion for small molecules and proteins.<br>
Page 6: Classes of Protein Translocation
Two classes of translocation:
Diffusion and Translocation
N-terminal signal sequence helps in localization.
Nuclear localization signals guide nuclear proteins via importins.
Post-translational and co-translational translocation methods discussed.
Page 7: Nuclear Pores
Structure of nuclear pores facilitates import and export of proteins.
Large protein complexes and nuclear localization signals are essential for transport.
Examples of proteins that function in the nucleus are mentioned.
Page 8: Imported Proteins
Fully folded proteins are imported via nuclear import receptors.
Similar receptors exist for RNA export from the nucleus.
Page 9: Nuclear Pore Function
Receptor-cargo complexes navigate nuclear pore proteins through a gel-like mesh.
Page 10: Recent Pore Structures
Structures of nuclear pores discussed with references provided.
Page 11: Protein Transport to Mitochondria and Plastids
Post-translational sorting with signal peptides recognized by transport channels.
Page 12: Mitochondrial Protein Import
Proteins synthesized in the cytoplasm remain unfolded while being imported into mitochondria.
Page 13: Targeting Proteins to Thylakoids
Proteins must be recognized by sequences and cleaved post-insertion in thylakoids.
Page 14: Example - Thylakoid Protein Import
Description of how proteins cross membranes through two channels in thylakoids.
Page 15: Endomembrane System Overview
The endomembrane system's various components like nucleas, plasma membrane, golgi, and ER.
Page 16: Endomembrane System Continuity
The nuclear lumen is contiguous with the rough ER; protein synthesis begins in the cytosol.
Page 17: Signal Sequences and Translation
Signal sequences guide the nascent protein to the rough ER and partner with SRPs for transport; translation proceeds simultaneously.
Page 18: SRPs and Translocation
SRPs mediate attachment to the ER membrane via receptors and translocon for protein passage.
Page 19: Role of Signal Sequence
Signal sequences enable channel opening and are cleaved post-translocation, allowing protein folding.
Page 20: ER Transmembrane Proteins
Additional start/stop transfer sequences regulate transmembrane protein integration into membranes.
Page 21: Topologies of Multi-pass Proteins
Design of transmembrane proteins is influenced by internal sequences determining orientation.
Page 22: Golgi Budding and Cargo Transport
Mechanism of vesicles from ER to Golgi is detailed, emphasizing microtubule facilitation.
Page 23: Sorting in the Endomembrane System
Similar destination signal sequences cause sorting in the ER and Golgi.
Page 24: Quiz on Endomembrane System
Question posed regarding identifying components of the endomembrane system.
Page 25: Endomembrane Synthesis and Sorting
Proteins synthesized in rough ER, processed, modified and sorted in Golgi with recommended resources for learning.
Page 26: Organellar Genomes and Import Mechanism
Organelles have reduced genomes; nuclearly-encoded proteins are required for their function.
Page 27: Gene Migration to the Nucleus
Most mitochondrial genes migrated to the nuclear genome; implications discussed.
Page 28: Expression Differences in Gene Transfer
Differences between prokaryotic and eukaryotic gene expression highlight challenges following transfer.
Page 29: Expression Hurdles
Steps required to express transferred bacterial genes within eukaryotic cells outlined.
Page 30: Current Transfer Mechanisms
Modern processes of gene transfer into the nucleus and necessary adaptations are discussed.
Page 31: Gene Transfer Theories
Various scenarios of gene migration from mitochondrion to nucleus considered.
Page 32: Reasons for Gene Migration
Hypotheses explored regarding the motivation for organellar genes transitioning to the nucleus.
Page 33: DNA Sequence Changes
Different sequences highlighted for redundancy without functional change.
Page 34: Mutation Impact
Discussion on the effects of specific mutations, corroborated with examples.
Page 35: Functional Impact of Amino Acid Changes
Assessing consequences of mutation changes within proteins, emphasizing conditions for impact.
Page 36: Knowledge Application
Considerations for understanding mutation impacts based on protein function.
Page 37: Case Study - Mutations with Effects
White tiger mutation leading to loss of transporter function as specific example.
Page 38: Nonsense Mutations
Nonsense mutation results in incomplete protein synthesis, outlining genetic implications.
Page 39: Mutation Questions
Queries surrounding the potential dominance of nonsense mutations discussed.
Page 40: Frame-shift Mutations
Consequences of insertions and deletions on amino acid sequence presentation.
Page 41: Frameshift Effects Explained
Simple explanations on how frameshift mutations can cause widespread effects on protein structure.
Page 42: Mitigation of Insertions/Deletions
Potential resiliency of certain insertion/deletion mutations addressed.
Overview of Protein Translocation and Organellar Genomes
Learning Goals: Understand how nuclearly-encoded proteins reach their destinations, including cytoplasmic, nuclear, organellar, and exported proteins.
Protein Synthesis: Cytosolic proteins remain in the cytosol, while others need membrane embedding. Nuclearly-encoded proteins can't pass through lipid bilayers directly.
Nuclear Protein Transfer: Requires crossing the nuclear membrane via nuclear pores, permitting small molecule diffusion.
Translocation Classes: Involves diffusion and translocation, with N-terminal signals and nuclear localization signals guiding proteins.
Mitochondrial and Plastid Import: Proteins are synthesized in the cytoplasm in an unfolded state before import, needing specific signal peptides.
Endomembrane System: Comprises nuclear, plasma membrane, Golgi, and ER; proteins synthesized in rough ER are processed in the Golgi.
Gene Migration: Most mitochondrial genes migrated to the nuclear genome, impacting expression in eukaryotic cells and presenting challenges.