Membrane Fusion

Membrane Fusion

Author: Judith M. White

Source: Science, New Series, Vol. 258, No. 5084 (Nov. 6, 1992), pp. 917-924

Published by: American Association for the Advancement of Science

Stable URL: http://www.jstor.org/stable/2881663 .

Introduction• Ubiquitous cell biological process• Part of many house keeping functions– Endocytosis– Constitutive secretion– Recycling

• Specialization of cells– Sperm-egg fusion– Exocytosis of hormones,

neurotransmitters, enzymes

Introduction• Role in disease– Giant cells during

inflammatory response– Viral entry into host cells– Induced cell-cell fusion by

HIV• Several types of fusion

events

Influenza HA-Mediated Membrane Fusion

• Reasons for being a model for study– Single gene product– Easily induced by pH change– Easily produced– X-ray structure known– Large database of cloned

sequences– Numerous characterized

mutants

Influenza HA-Mediated Membrane Fusion

• Hemagglutinin (HA)– Trimer of identical subunits– Conserved, hydrophobic

fusion peptide in stem of each subunit• Slight mutations in structure

alter or abolish fusion capabilities

– Receptor binding sites lie at distal tip of protein (globular head)

Influenza HA-Mediated Membrane Fusion

• Fusion Steps1. Fusion peptides, induced by low pH, destabilize viral and

target bilayers2. Conformational changes in HA trimer (rotational and lateral

movements) after initial binding to receptor3. Hemifusion followed by full fusion to form fusion pore

Exocytosis of Mast Cell Granules• Fusion of intracellular storage vesicle with the cytoplasmic

face of the plasma membrane.• The mast cell releases a mixture of compounds, including

histamine, proteoglycans, serotonin, and serine proteases from its cytoplasmic granules. (inflammatory response to allergen)

Exocytosis of Mast Cell Granules• Fusion Steps

1. Outward current transient (release of Ca2+)—discharge of vesicle membrane potential through nascent pore

2. Increase in cell membrane capacitance due to interaction with vesicle (directly proportional to surface area

3. Narrow conducting pore forms (2-2.5 nm)—width of 10 water molecules

4. Pore grows in concurrence with capacitance until it reaches a plateau level

Intra-Golgi Transport

• Membrane fusion essential for membrane trafficking of macromolecules

• Studied through careful application of inhibitors

Intra-Golgi Transport• Steps in Transportation Between Cisterna

1. Nascent coated bud forms on the donor cisterna and detaches

2. Targeted and attaches to acceptor cisterna3. Vesicle is uncoated and matures4. Fuses with acceptor cisterna5. Deposits contents

Intra-Golgi Transport• Requirements– ATP and GTP for vesicle

budding– GTP hydrolysis for uncoating– Fatty Acyl coA for budding

and maturation– Ca2+ cofactor for late

transport– GTPases (monomers and

trimers) and phosphoproteins for fidelity, timing, and vector

Intra-Golgi Transport• Fusion Machine– 3 Proteins• Homotetramer NSF (NEM-sensitive factor)• SNAP (Soluble NSF Attachment Protein)• Integral Membrane Receptor (SNARE)

Membrane Fusion• Still a lot to learn!!!• Pores developed during influenza-mediated fusion and

exocytosis of mast cell granules• Complex fusion machine utilized during intra-Golgi transport

(unknown if pore forms)• Why is this complexity necessary for membrane fusion

events to occur??• It is possible that the fusion pore is element that units all

fusion events—viral and cell-cell fusion, regulated exocytosis, intra-Golgi transport, and other intracellular fusion events

For the exam…

• Identify when membrane fusion occurs in cells• Why is it necessary?