Biogenesis: How Proteins Are Made and Transported in Cells

Central Dogma of Molecular Biology

Overview of Protein Synthesis

The central dogma of molecular biology describes the flow of genetic information within a cell, specifically how proteins are synthesized. This process is fundamental to all living organisms and involves the transcription of DNA into RNA and the translation of RNA into proteins.

• DNA contains genetic instructions for making one or more proteins.

• Transcription is the process by which DNA is copied into messenger RNA (mRNA), a mobile form of genetic information.

• In prokaryotes, transcription and translation occur in the cytoplasm.

• In eukaryotes, transcription occurs in the nucleus, and translation occurs in the cytoplasm.

• Translation is the process by which ribosomes "read" mRNA instructions to synthesize proteins.

Example: The majority of proteins (>99%) are made in the cytoplasm, except for those synthesized in mitochondria and chloroplasts.

Cellular Localization of Proteins

How Proteins End Up in Specific Cell Locations

Proteins must be correctly localized within the cell to perform their functions. This is achieved through cellular structures and transport mechanisms.

• Cytoplasm: Some proteins remain in the cytoplasm.

• Plasma Membrane: Proteins can be embedded in or transported across the plasma membrane.

• Secretion: Cells move materials to the outside via secretion.

• Organelles: Eukaryotic cells contain different organelles, each with a unique set of proteins and functions.

Example: The nucleus, mitochondria, and endoplasmic reticulum each have distinct protein compositions.

Protein Targeting and Transport

Unique Functions and Protein Sorting

Proteins are sorted and transported to specific cellular locations to ensure each cell structure can perform its unique function. Two major systems are involved: nuclear transport and the endomembrane system.

• Proteins may remain in the cytoplasm or be transported to organelles.

• Transport systems ensure proper protein localization.

Nuclear Transport

Structure and Function of the Nucleus

The nucleus is surrounded by the nuclear envelope, which consists of two layers:

• Inner membrane: Faces the inside of the nucleus.

• Outer membrane: Faces the cytoplasm.

The nuclear envelope contains nuclear pores, which are protein-based structures that regulate the movement of materials between the nucleus and cytoplasm.

• Nucleolus: A region within the nucleus where ribosomes are assembled.

• Ribosomal subunits exit the nucleus through nuclear pores to reach the cytoplasm.

Nuclear Pore Complex and Selective Transport

The nuclear pore complex controls the entry and exit of proteins and other molecules. Not all proteins travel through the pore; some must remain in the cytoplasm.

• Central plug: Acts as a gate, closing the pore when necessary.

• Transport through the pore is selective and regulated by signals.

Example: Proteins with a nuclear import signal can enter the nucleus, while those with a nuclear export signal can exit.

Key Terms and Structures

• Nuclear envelope: Double membrane surrounding the nucleus.

• Nuclear pore: Channel for molecular transport.

• Nucleolus: Site of ribosome assembly.

• Central plug: Gate mechanism in the nuclear pore.

Summary Table: Protein Localization Signals

Protein Type	Signal Present	Localization
Stays in cytoplasm	None	Cytoplasm
Permanently in nucleus	Nuclear import signal	Nucleus
Moves between nucleus and cytoplasm	Import and export signals	Both compartments
Exported from nucleus (e.g., mRNA)	Nuclear export signal	Cytoplasm

Additional info:

• ATP and oxygen are required for active transport through nuclear pores.

• Proteins and ribosomes are selectively transported based on signal sequences.