Structure (10; 110) of a body side panel of a motor vehicle, comprising at least: - a first front structural upright (12; 112) or front pillar, - a second central structural upright (13; 113) or central pillar which, with said first front structural upright (12; 112), delimits a first opening (14; 114) intended to receive a first front side door, - a third rear structural upright (15; 115) or rear pillar which, with said second central structural upright (13; 113), delimits a second opening (16; 116) intended to receive a second rear side door, - a lower longitudinal member (17; 117) or closing member which, with said first (12; 112), second (13; 113) and third (15; 115) structural uprights, delimits at the bottom said first (14; 114) and second (16; 116) openings for said associated side doors and comprises: ∘ a first front portion (18; 118) or main portion, comprising a first end (18a; 118a) connected to said associated first front structural upright (12; 112), and a second end (18b; 118b) arranged at a connecting zone (20; 120) between said central (13; 113) and rear (15; 115) structural uprights, ∘ a second rear portion (19; 119) or interchangeable portion, comprising a first end (19a; 119a) connected to the first front portion (18; 118) at said connecting zone (20; 120), and a second end (19b; 119b) connected to said associated third rear structural upright (15; 115), which structure (10; 110) is characterized in that said interchangeable portion (19; 119) of said lower longitudinal member (17; 117) comprises a predefined length (L1, L2) depending on the structure (10; 110) associated with the type of motor vehicle concerned, and in that said second rear portion (19; 119) of said lower longitudinal member (17; 117) is made of soft steel, and in that said first front portion (18; 118) of said lower longitudinal member (17; 117) is made of steel with a very high elastic limit. - a first front structural upright (12; 112) or front pillar, - a second central structural upright (13; 113) or central pillar which, with said first front structural upright (12; 112), delimits a first opening (14; 114) intended to receive a first front side door, - a third rear structural upright (15; 115) or rear pillar which, with said second central structural upright (13; 113), delimits a second opening (16; 116) intended to receive a second rear side door, - a lower longitudinal member (17; 117) or closing member which, with said first (12; 112), second (13; 113) and third (15; 115) structural uprights, delimits at the bottom said first (14; 114) and second (16; 116) openings for said associated side doors and comprises: ∘ a first front portion (18; 118) or main portion, comprising a first end (18a; 118a) connected to said associated first front structural upright (12; 112), and a second end (18b; 118b) arranged at a connecting zone (20; 120) between said central (13; 113) and rear (15; 115) structural uprights, ∘ a second rear portion (19; 119) or interchangeable portion, comprising a first end (19a; 119a) connected to the first front portion (18; 118) at said connecting zone (20; 120), and a second end (19b; 119b) connected to said associated third rear structural upright (15; 115), ∘ a first front portion (18; 118) or main portion, comprising a first end (18a; 118a) connected to said associated first front structural upright (12; 112), and a second end (18b; 118b) arranged at a connecting zone (20; 120) between said central (13; 113) and rear (15; 115) structural uprights, ∘ a second rear portion (19; 119) or interchangeable portion, comprising a first end (19a; 119a) connected to the first front portion (18; 118) at said connecting zone (20; 120), and a second end (19b; 119b) connected to said associated third rear structural upright (15; 115),