The use of influenza virus labelled with radio-sulpher in studies of the early stages of the interaction of virus with the host cell

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Abstract
1. Influenza virus can be labelled with 35S by cultivation in fertile eggs into which radioactive methionine has been introduced.2. When virus labelled with 35S is fractionated by ether treatment the nucleoprotein soluble antigen fraction contains 15 % of the 35S, the haemagglutinin also contains 15%, while the denatured envelope protein contains 70% of the 35S.3. When sulphur-labelled virus is introduced as a primary inoculum in fertile eggs and extracts of the chorioallantoic membrane are made 1–20 % of 35S is recovered as amino-acid, while the remainder is present as non-infective material of particle size similar to that of the original virus.4. It is suggested that on entry into the cell the virus nucleoprotein is hydrolysed with the release of amino-acid and free nucleic acid, while the virus envelope protein and haemagglutinin remain on the cell surface.The authors are indebted to the Medical Research Council for a grant in aid of expenses, and to Messrs. Eli Lilly and Co. for the generous provision of the Spinco centrifuge used in this work. The radioactive methionine was obtained from the Radiochemical centre at Amersham.