A man accused of badly beating a teenage actor on a San Francisco bus has pleaded not guilty to felony assault and robbery charges.

Prosecutors say 18-year-old Uluao Mase was carrying a loaded gun when he and three others beat 18-year-old Christopher Borgzinner and stole his wallet and iPod. Two others suspects, ages 15 and 16, are also in custody. Police are seeking a fourth.

Police say Borgzinner was going to an acting class when the suspects asked if his red sneakers meant he was in a gang. Despite replying no, police say the suspects pummeled Borgzinner, who sustained fractures under both eyes.

Borgzinner plays a gang member in "La Mission," a new film starring Benjamin Bratt.

Mase is being held on $200,000 bail.