LOS ANGELES -- USC interim coach Clay Helton's quest to earn the coveted job gained a healthy dose of momentum Saturday afternoon after the unranked Trojans upset No. 22 UCLA 40-21 in front of a crowd of 83,602 at the Coliseum.

USC (8-4, 6-3) won the Pac-12 South and will play Pac-12 North champion Stanford for the conference championship Dec. 5 at Levi's Stadium in Santa Clara. A victory would send USC to the New Year's Day Rose Bowl for the first time since 2009.

"I cannot tell you how proud I am of this group of men, specifically the seniors and the team as a whole," Helton said. "I think about the seniors and the rollercoaster ride they have gone through. For them to call themselves Pac-12 South champions is a victory."

But now does USC athletic director Pat Haden turn the keys to this Trojans football program over to Helton?

"That is not my job," Helton said. "My job is to watch over this team and win every game we are associated with."

The 85th renewal of this crosstown rivalry contained enough dramatic turns and twists to create yet another exciting moment in this storied series.

One frantic fan was heard to exclaim, "This game's going to give me a heart attack!" His remark came in the aftermath of the game's sixth lead change, which occurred in the quarter when USC defensive end Rasheem Green rambled 31 yards to the end zone with a fumble to give the Trojans a 26-21 lead.

Green's first career touchdown came mere minutes after UCLA running back Paul Perkins plunged in from the 1-yard line to give UCLA a 21-20 lead.

USC led 3-0 on a 30-yard field goal by Alex Wood. Perkins scored the first of his two touchdowns to give UCLA a 7-3 first-quarter lead. USC quarterback Cody Kessler's 1-yard sneak, three minutes into the second quarter, gave the lead back to USC at 10-7. But the Bruins regained the advantage at 14-10 on a 19-yard pass from freshman quarterback Josh Rosen to wide receiver Thomas Duarte, with 9:49 left in the first half.

Wood kicked another field goal to narrow the margin to 14-13 and then a breathtaking 42-yard punt return for a touchdown by sophomore Adoree' Jackson gave USC the halftime lead at 20-13.

"We knew this one was going to be tough," Kessler said. "We made a decision, either Monday or Tuesday this week, that we weren't losing this game. We rallied behind coach Helton. He always gets us going."

Perkins' touchdown, which catapulted UCLA back in front, was the final gasp for the Bruins. USC exerted its physical will on both sides of the ball to wear down a UCLA team that saw its three-game winning streak in the series come to a stop.

"There's a tremendous amount of disappointment in our locker room," UCLA coach Jim Mora said. "They did everything they could to play their best and we just didn't perform as well as we needed to get the W."

Fifth-year senior Kessler went out in style in the final home game of his career. Kessler's statistics were not eye-popping. He completed 15 of 26 passes for only 175 yards. But Kessler threw a pair of touchdown passes and ran for another. His offensive line -- which has undergone much change because of injury -- protected him well. Kessler was sacked just once.

"The run game was going to be a huge part of trying to establish ourselves as a physical football team," Helton said. "And we exhibited that today."

USC players endorsed Helton for the job and the removal of his interim status. Helton added to his resume by improving to 5-2 since taking over from Steve Sarkisian, who was fired two days after USC's loss to Washington.

UCLA (8-4, 5-4) will have a few weeks to rest, soothe its physical and emotional wounds, and then prepare for a bowl game.

"They'll bounce back," Mora said. "They're resilient and they'll press on and be the best that they can be. We'll lick our wounds and come back stronger than ever."

Duarte said, "It all hurts. It's disappointing. It's like a bad dream, really. Everything was right there for us and we didn't execute."

Rosen continued to show that he could develop into one of the nation's best quarterbacks. Rosen, who started all 12 games, completed 19 of 37 passes for 227 yards and one touchdown. But his school-record streak of 246 pass attempts without an interception was snapped when USC freshman cornerback Iman Marshall picked off a pass in the third quarter. Marshall later intercepted a second pass.

"We made the (USC) defense look a little better than they necessarily were," Rosen said. "It's no shot at them. We just didn't execute."

UCLA has prided itself on being more physical, especially up front, since Mora took over as coach three years ago. But the Bruins were worn down and shut out in the fourth quarter by a bruising USC offensive line that paved the way for 235 rushing yards.

USC's running game has thrived behind a trio of backs, led by freshman Ronald Jones II, who has established a school record for a first-year Trojan with 890 yards. Senior Tre Madden has two 100-yard rushing games this year. On Saturday it was junior Justin Davis' turn to carry the load; he ran for 130 yards on 25 carries, his second week in a row over 100 yards.

NOTES: Adoree Jackson's punt return was his second in two weeks. He also scored on a punt return in USC's loss to Oregon. In only two seasons, Jackson has scored touchdowns on an interception return, a punt return, a kickoff return and a pass reception. ... UCLA senior Jordan Payton, the Bruins' career receiving leader, caught four passes for 60 yards and extended his streak to 28 consecutive games with at least one catch. ... USC sophomore WR JuJu Smith-Schuster added to his credentials for All-American honors by catching six passes for 85 yards. Smith-Schuster now has 74 catches for 1,302 yards and 10 touchdowns. ... UCLA junior LB Jayon Brown finished with a game-high 13 tackles. Brown was high school teammates at Long Beach Poly with fellow Bruins DB Randall Goforth and with Smith-Schuster and USC freshman CB Iman Marshall.