Sometimes the story is in knowing how to connect the dots (or electron micrographs). In 1978, Ari Helenius took his laboratory—which had already made a name for itself in isolating and characterizing membrane proteins—in a new, cellular direction. He emerged with a biological storyboard composed of electron microscopy (EM) snapshots.

graphic

Semliki Forest virus enters cells in endosomes.

HELENIUS

Helenius wanted to figure out how viruses enter cells—an important question from the literature—but admits that starting out he “didn't have the mindset of a cell biologist.” The individual results of those first experiments following Semliki Forest virus (SFV) into cultured mammalian cells by immunofluorescence and EM were clear, Helenius says, “but I couldn't put them in context.”

A meeting in Berlin in the spring of 1978 helped change his perspective. There he rubbed elbows with “the big cell biology crowd,” including George Palade, Christian de Duve, Michael Brown, and Joseph Goldstein and got a “five-day infusion” of current cell biology thinking. “I came back to lab and a few weeks later in one single moment on a Thursday afternoon at about 5:15 everything fell into place.”

What Helenius could now see in his data was that SFV entered the cell through clathrin-coated pits and continued on by endocytosis to lysosomes (Helenius et al., 1980). By treating the cells with chemicals that raised the pH of the lysosome, the team also showed that the acidic pH of the endocytic vacuoles was needed to induce the fusion that released the virus into the cytoplasm of the cell.

“Everyone thought this was a dead-end, degradative pathway,” says Judith White, who joined the lab as a post-doc shortly after the work. “This work was so good it finally convinced people.” The work also set off a number of major new research initiatives. Several labs went on to explore how other viruses enter cells and collectively found that about two-thirds of animal viruses use the same endocytic pathway and similar overall mechanisms.

The paper's in vitro section showed that adding acid to a preparation of liposomes and isolated SFV induced fusion. This inspired a new lab system for studying synchronized membrane fusion events (White et al., 1980). In addition, the observation that SFV entered the cell's cytoplasm before actually reaching the lysosome hinted that another slightly acidic compartment existed between the plasma membrane and the lysosome. A few years later, the Helenius lab defined that compartment as what would come to be known as the late endosome (Marsh et al., 1983).

But above all this, the paper bolstered the idea that most fascinates Helenius: viruses have evolved a number of sophisticated strategies to exploit the functions of their host. “They are microscopic Trojan horses,” he says. “Using information they have incorporated into their structure over millions of years of coevolution, viruses know the cellular passwords and pin codes.”

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