The hydrophobic photoaffinity label 3-(trifluoromethyl)-3-(m- [',sI] (14), it is our contention that this approach provides a unique opportunity to identify the fusion-initiating proteins and permits greater insight into the mechanisms of viral entry and membrane fusion.The infectious entry of enveloped viruses is accomplished by a mechanism involving membrane fusion (1-3). Sendai virus, a paramyxovirus, enters host cells by fusion of the viral envelope with the cell's plasma membrane, mediated by the two Sendai envelope glycoproteins (1, 3). The hemagglutinin/neuraminidase (HN) mediates viral attachment to sialic acid-containing cell surface receptors, while the fusion (F) protein, which consists of two disulfide-linked subunits, F1 and F2 (4), triggers the actual fusion reaction. It has been proposed that fusion is initiated as a result of the insertion of the hydrophobic F1 NH2 terminus, consisting of about 20 amino acids, into the target membrane (1, 3, 5, 6).Hydrophobic protein-lipid interactions (7-10) and some proteins that cause membrane fusion (11, 12) have been investigated by using photoaffinity labels. Such studies typically involve labeling of both protein and lipid after an incubation period, allowing identification of the transmembrane segments of proteins or a distinction between subunits potentially interacting with membranes. Protein-induced fusion involves an initial local interaction between fusogen and apposed membranes, rapidly followed by randomization of membrane components in the lateral plane of the newly formed (i.e., fused) membrane. By focusing on the very early events at the onset of fusion-i.e., those prior to membrane randomization-the proteins penetrating the target membrane as fusion initiators can be selectively labeled. In the case of Sendai virus fusion, such an experiment would allow analysis of the hypothesis that fusion is initiated by insertion of the hydrophobic F1 NH2 terminus into the target membrane. In order to examine exclusively these initial interactions, photolabeling must be done for limited periods of time-i.e., while fusion is in progress, before the proteins have reoriented in the fused membrane. Obviously, this requires a detailed knowledge of the kinetics of the fusion reaction. ITo whom reprint requests should be addressed.

Bacteriophage lambda attaches to Gram-negative bacteria using the outer membrane protein LamB as its receptor. Subsequently, DNA is injected by the bacteriophage into the host cell for replication and expression. The mechanism of DNA injection, however, is poorly understood. In order to begin to characterize DNA injection, a quantitative kinetic assay to detect injection into reconstituted LamB liposomes is described. The technique involves monitoring the increase in fluorescence of liposome-encapsulated ethidium bromide, which occurs as DNA enters the aqueous compartment of the vesicles. The data indicate that injection is several times faster than indicated by earlier studies and is complete within 1 min. Such assays which allow direct observation of this process are necessary first steps toward a mechanistic understanding.

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We present an unusual case of a granulocytic sarcoma (chloroma) of the sacrum which predated the initial clinical manifestation of acute myelogenous leukemia. Although granulocytic sarcomas occur in up to 9.1% of cases of acute myelogenous leukemia they usually present concurrently with the leukemic presentation. Although granulocytic sarcomas can involve several different organ systems, bone is the most common site.