Protein-protein interactions

Masoud Youssefi, MD,PhDDivision of microbiology/virology

Introduction• important field in cell biology, biochemistry Localization and trafficking posttranslational modifications signaling networks • also important field in viral replication• very difficult to predict• two main patterns: ■ domain-domain interactions ■domain-peptide interactions

An example: virion assembly

• The components come together and the Nucleocapsid is formed which in turn will become completed to the whole particle.

• The assembly process begins when concentration of structural proteins is enough within the cell to drive the process.

• Many protein-protein, protein-nucleic acid and in case of membrane viruses protein-membrane (fatty acid) interactions are needed.

3

The mechanism of interaction

• Non-covalent so reversible• Van del waals forces• Hydrophobic interactions• Electrostatic bonds• Hydrogen bonds• For strong couplings very accurate force field

potentials are needed

How to study protein protein interaction?

Overview of techniques

• Gel filtration• Far western blot• Affinity

chromatography• Co-

immunopercipitation• Capillary elecrophoresis• Biosensor

• FRET microscopy• Confocal microscopy• 2 hybrid assay• Protein microarry• Maspec• NMR• Co-crystallization for

crystallography

Gel filtration chromatography

• Also called ”Size exclusion”• Porous made up of cross-

linked polymers• Small molecules are

trapped by the beads• For self assembling proteins

monomers come later

Far western blot• Also called ”Blot overlay”• Fractionating proteins on

SDS-PAGE• Blotting to nitocellulose or

PVDF membrane• Overlaying with a solution

of the protein of interest• Binding the added protein

to an immobilized protein on the membrane

• Detection with antibody against the overlaying protein

Co-Immunoprecipitation• Protein A binds to

antibodies• Sepharose beads coated

with protein A• Specific antibody binds to

the protein of interest • The complex is precipitated

by binding to the beads via protein A

• Proteins are released from beads by boiling

• Western blot

Affinity chromatography

• In the case of His- tagged proteins• The His-tagged protein binds to nickel or

cobalt column • His-tagged protein and it’s associated protein

are eluted from the column by adding imidazole

Fluorescence resonance energy transfer (FRET)

FRET cont• Cyan fluorescence protein (CFP) and yellow

fluorescence protein (YFP) are spectral variants of GFP

• Plasmid constructs to fuse the proteins of interest to CFP and YFP

• Co-transfection of plasmids to the cells• Fixation of the cells and view by confocal microscopy• Disadvantage:False negative results: If the fluorophores are over 200Ǻ apart while the

proteins interact with each other, no signal will be observed

FRET using CFP & YFP

Fluorescence resonance energy transfer (FRET)

Yeast two-hybrid assay

Yeast two hybrid assay

• Transcription factor, Gal4p, has DNA binding (BD)(aa1-147) and transcriptional activator(AD)(aa768-881) domains

• Stimulates transcription at a promoter reconized by Gal4p (upstream activating sequence,UAS)

• Lac Z reporter gene encodes beta-galactosidase which produces blue pigment when the colony is grown in a media containing X-Gal

• Disadvantage:time consuming!

2 Hybrid system

Mamalian two-hybrid assay• Is analogous to Y2H assay• Plasmids: 1)Gal4pBD-fusion vector 2)VP16AD-fusion vector(viral activator) 3)luciferase reporter plasmid contaning multiple copies of Gal4p binding sites(UAS)

• Co-transfection: in the case of interaction, luciferase activity will be detected

• Advantage: good for studying mammalian proteins: they may not fold correctly in yeast or they may require post-tranlational modifications for protein interaction

what are biosensors?

• Transducer converts physical change(heat, change in charge, light absorbance, mass) into an electrical signal

Confocal microscopy

• A good technique to detect intracellular co-localization of proteins

• Point scan laser system minimizes overlaps in image (perfect for imaging Co-localization of proteins)

Confocal microscopy cont.

Overview of techniques

• Gel filtration• Far western blot• Affinity

chromatography• Co-

immunopercipitation• Capillary elecrophoresis

• FRET microscopy• Confocal microscopy• 2 hybrid assay• Maspec• NMR• Co-crystallization for

crystallography

• Thank you!