"Toniu has petitioned to enter the draft," Nebraska coach Frank Solich said. "We are sorry to see Toniu go, but we certainly wish him the best in his NFL endeavors."
The 6-4, 340-pound Fonoti set a school record with 201 so-called "pancake" blocks this season. A native of Hawaii, he has been projected as a first-round pick by some scouts, although he is considered a better run blocker than pass protector.
Williams had 101 tackles last season, including 10 for losses.
Washington called a news conference for Thursday after telling The Tennessean that he will leave for the NFL after just one season of college football.
"I'm what the NFL wants right now. They're all looking for big physical receivers, and I think I'm at the top of the list," the 6-4, 225-pound Washington told the newspaper. "I don't think my draft position would change much from this year to next."
The 22-year-old Washington came to Tennessee as a walk-on after four years of minor league baseball. He caught 64 passes for 1,010 yards and five touchdowns this season, including a school-record 256 receiving yards in a regular-season win over Louisiana State.
Washington's teammate, defensive tackle Albert Haynesworth, announced Tuesday he would enter the NFL draft.
The expansion Houston Texans hold the first pick in the draft, followed by Carolina and Detroit.
Players have until Friday to formally apply for the draft, which will be held April 20-21 in New York.